diff --git a/core/lib/Drupal/Core/Extension/Extension.php b/core/lib/Drupal/Core/Extension/Extension.php
index 49ca6c2db173..84792b976c84 100644
--- a/core/lib/Drupal/Core/Extension/Extension.php
+++ b/core/lib/Drupal/Core/Extension/Extension.php
@@ -63,6 +63,8 @@ class Extension {
    *   (optional) The filename of the main extension file; e.g., 'node.module'.
    */
   public function __construct($root, $type, $pathname, $filename = NULL) {
+    // @see \Drupal\Core\Theme\ThemeInitialization::getActiveThemeByName()
+    assert($pathname === 'core/core.info.yml' || ($pathname[0] !== '/' && file_exists($root . '/' . $pathname)), sprintf('The file specified by the given app root, relative path and file name (%s) do not exist.', $root . '/' . $pathname));
     $this->root = $root;
     $this->type = $type;
     $this->pathname = $pathname;
@@ -155,7 +157,7 @@ class Extension {
    */
   public function __call($method, array $args) {
     if (!isset($this->splFileInfo)) {
-      $this->splFileInfo = new \SplFileInfo($this->pathname);
+      $this->splFileInfo = new \SplFileInfo($this->root . '/' . $this->pathname);
     }
     return call_user_func_array([$this->splFileInfo, $method], $args);
   }
diff --git a/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php b/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
index 477f74fbe732..eb0278950aa6 100644
--- a/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
+++ b/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
@@ -47,6 +47,7 @@ EOF;
     vfsStream::create([
       'modules' => [
         'test_module' => [
+          'test_module.info.yml' => $test_module_info,
           'tests' => [
             'src' => [
               'Functional' => [
@@ -212,7 +213,7 @@ EOF;
 
     $test_discovery = new TestDiscovery('vfs://drupal', $class_loader->reveal(), $module_handler->reveal());
 
-    $result = $test_discovery->getTestClasses(NULL, ['PHPUnit-Kernel']);
+    $result = $test_discovery->getTestClasses('test_profile_module', ['PHPUnit-Kernel']);
     $expected = [
       'example3' => [
         'Drupal\Tests\test_profile_module\Kernel\KernelExampleTest4' => [
diff --git a/core/modules/system/tests/src/Unit/Menu/SystemLocalTasksTest.php b/core/modules/system/tests/src/Unit/Menu/SystemLocalTasksTest.php
index 3123ca616b2e..fa17be8ad196 100644
--- a/core/modules/system/tests/src/Unit/Menu/SystemLocalTasksTest.php
+++ b/core/modules/system/tests/src/Unit/Menu/SystemLocalTasksTest.php
@@ -31,7 +31,7 @@ class SystemLocalTasksTest extends LocalTaskIntegrationTestBase {
 
     $this->themeHandler = $this->createMock('Drupal\Core\Extension\ThemeHandlerInterface');
 
-    $theme = new Extension($this->root, 'theme', '/core/themes/bartik', 'bartik.info.yml');
+    $theme = new Extension($this->root, 'theme', 'core/themes/bartik', 'bartik.info.yml');
     $theme->status = 1;
     $theme->info = ['name' => 'bartik'];
     $this->themeHandler->expects($this->any())
diff --git a/core/modules/user/tests/src/Unit/PermissionHandlerTest.php b/core/modules/user/tests/src/Unit/PermissionHandlerTest.php
index 86455b673c5b..6964ac95b0bc 100644
--- a/core/modules/user/tests/src/Unit/PermissionHandlerTest.php
+++ b/core/modules/user/tests/src/Unit/PermissionHandlerTest.php
@@ -76,7 +76,7 @@ class PermissionHandlerTest extends UnitTestCase {
    *   The extension object.
    */
   protected function mockModuleExtension($module, $name) {
-    $extension = new Extension($this->root, $module, "modules/$module");
+    $extension = new Extension('vfs:/', $module, "modules/$module");
     $extension->info['name'] = $name;
     return $extension;
   }
diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
index fcea71b3da0e..ad188a4ed34b 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
@@ -26,7 +26,7 @@ class ExtensionListTest extends UnitTestCase {
    */
   public function testGetNameWithNonExistingExtension() {
     list($cache, $info_parser, $module_handler, $state) = $this->getMocks();
-    $test_extension_list = new TestExtension($this->root, 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal(), $state->reveal(), 'testing');
+    $test_extension_list = new TestExtension($this->randomMachineName(), 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal(), $state->reveal(), 'testing');
 
     $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
     $extension_discovery->scan('test_extension')->willReturn([]);
@@ -50,7 +50,7 @@ class ExtensionListTest extends UnitTestCase {
    */
   public function testGetWithNonExistingExtension() {
     list($cache, $info_parser, $module_handler, $state) = $this->getMocks();
-    $test_extension_list = new TestExtension($this->root, 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal(), $state->reveal(), 'testing');
+    $test_extension_list = new TestExtension($this->randomMachineName(), 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal(), $state->reveal(), 'testing');
 
     $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
     $extension_discovery->scan('test_extension')->willReturn([]);
@@ -142,7 +142,7 @@ class ExtensionListTest extends UnitTestCase {
 
     $filenames = $test_extension_list->getPathnames();
     $this->assertEquals([
-      'test_name' => 'vfs://drupal_root/example/test_name/test_name.info.yml',
+      'test_name' => 'example/test_name/test_name.info.yml',
     ], $filenames);
   }
 
@@ -153,7 +153,7 @@ class ExtensionListTest extends UnitTestCase {
     $test_extension_list = $this->setupTestExtensionList();
 
     $pathname = $test_extension_list->getPathname('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name/test_name.info.yml', $pathname);
+    $this->assertEquals('example/test_name/test_name.info.yml', $pathname);
   }
 
   /**
@@ -174,7 +174,7 @@ class ExtensionListTest extends UnitTestCase {
     $test_extension_list = $this->setupTestExtensionList();
 
     $path = $test_extension_list->getPath('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name', $path);
+    $this->assertEquals('example/test_name', $path);
   }
 
   /**
@@ -184,24 +184,24 @@ class ExtensionListTest extends UnitTestCase {
     $test_extension_list = $this->setupTestExtensionList();
 
     $path = $test_extension_list->getPath('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name', $path);
+    $this->assertEquals('example/test_name', $path);
     $pathname = $test_extension_list->getPathname('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name/test_name.info.yml', $pathname);
+    $this->assertEquals('example/test_name/test_name.info.yml', $pathname);
     $filenames = $test_extension_list->getPathnames();
     $this->assertEquals([
-      'test_name' => 'vfs://drupal_root/example/test_name/test_name.info.yml',
+      'test_name' => 'example/test_name/test_name.info.yml',
     ], $filenames);
 
     $test_extension_list->reset();
 
     // Ensure that everything is still usable after the resetting.
     $path = $test_extension_list->getPath('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name', $path);
+    $this->assertEquals('example/test_name', $path);
     $pathname = $test_extension_list->getPathname('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name/test_name.info.yml', $pathname);
+    $this->assertEquals('example/test_name/test_name.info.yml', $pathname);
     $filenames = $test_extension_list->getPathnames();
     $this->assertEquals([
-      'test_name' => 'vfs://drupal_root/example/test_name/test_name.info.yml',
+      'test_name' => 'example/test_name/test_name.info.yml',
     ], $filenames);
   }
 
@@ -226,7 +226,7 @@ class ExtensionListTest extends UnitTestCase {
 
     list($cache, $info_parser, $module_handler, $state) = $this->getMocks();
     $info_parser->parse(Argument::any())->will(function ($args) {
-      return Yaml::decode(file_get_contents($args[0]));
+      return Yaml::decode(file_get_contents('vfs://drupal_root/' . $args[0]));
     });
 
     $test_extension_list = new TestExtension('vfs://drupal_root', 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal(), $state->reveal(), 'testing');
@@ -234,7 +234,7 @@ class ExtensionListTest extends UnitTestCase {
     $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
     $extension_scan_result = [];
     foreach ($extension_names as $extension_name) {
-      $extension_scan_result[$extension_name] = new Extension($this->root, 'test_extension', "vfs://drupal_root/example/$extension_name/$extension_name.info.yml");
+      $extension_scan_result[$extension_name] = new Extension('vfs://drupal_root', 'test_extension', "example/$extension_name/$extension_name.info.yml");
     }
     $extension_discovery->scan('test_extension')->willReturn($extension_scan_result);
     $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionSerializationTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionSerializationTest.php
index f8118a3dfb0a..5f705d6ccad6 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ExtensionSerializationTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionSerializationTest.php
@@ -5,6 +5,7 @@ namespace Drupal\Tests\Core\Extension;
 use Drupal\Tests\UnitTestCase;
 use Drupal\Core\Extension\Extension;
 use Drupal\Core\DependencyInjection\ContainerBuilder;
+use org\bovigo\vfs\vfsStream;
 
 /**
  * Tests Extension serialization.
@@ -14,6 +15,23 @@ use Drupal\Core\DependencyInjection\ContainerBuilder;
  */
 class ExtensionSerializationTest extends UnitTestCase {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    vfsStream::setup('dummy_app_root');
+    vfsStream::create([
+      'core' => [
+        'modules' => [
+          'system' => [
+            'system.info.yml' => file_get_contents($this->root . '/core/modules/system/system.info.yml'),
+          ],
+        ],
+      ],
+    ]);
+  }
+
   /**
    * Tests that the Extension class unserialize method uses the preferred root.
    *
@@ -30,12 +48,12 @@ class ExtensionSerializationTest extends UnitTestCase {
     $this->assertFalse(defined('DRUPAL_ROOT'), 'Constant DRUPAL_ROOT is defined.');
     $container = new ContainerBuilder();
     // Set a dummy container app.root to test against.
-    $container->set('app.root', '/dummy/app/root');
+    $container->set('app.root', 'vfs://dummy_app_root');
     \Drupal::setContainer($container);
     // Instantiate an Extension object for testing unserialization.
     $extension = new Extension($container->get('app.root'), 'module', 'core/modules/system/system.info.yml', 'system.module');
     $extension = unserialize(serialize($extension));
-    $this->assertEquals('/dummy/app/root', $this->readAttribute($extension, 'root'));
+    $this->assertEquals('vfs://dummy_app_root', $this->readAttribute($extension, 'root'));
   }
 
   /**
@@ -47,7 +65,7 @@ class ExtensionSerializationTest extends UnitTestCase {
   public function testPublicProperties() {
     $container = new ContainerBuilder();
     // Set a dummy container app.root to test against.
-    $container->set('app.root', '/dummy/app/root');
+    $container->set('app.root', 'vfs://dummy_app_root');
     \Drupal::setContainer($container);
     $extension = new Extension($container->get('app.root'), 'module', 'core/modules/system/system.info.yml', 'system.module');
     // Assign a public property dynamically.
diff --git a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
index e2b4cb277a50..655c99add355 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
@@ -318,7 +318,7 @@ class ModuleHandlerTest extends UnitTestCase {
     $module_handler->addModule('module_handler_test_added', 'core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added');
     $this->assertTrue($module_handler->implementsHook('module_handler_test_added', 'hook'), 'Runtime added module with implementation in include found.');
 
-    $module_handler->addModule('module_handler_test_no_hook', 'core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added');
+    $module_handler->addModule('module_handler_test_no_hook', 'core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_no_hook');
     $this->assertFalse($module_handler->implementsHook('module_handler_test_no_hook', 'hook', [TRUE]), 'Missing implementation not found.');
   }
 
@@ -502,8 +502,8 @@ class ModuleHandlerTest extends UnitTestCase {
   public function testGetModuleDirectories() {
     $module_handler = $this->getModuleHandler();
     $module_handler->setModuleList([]);
-    $module_handler->addModule('module', 'place');
-    $this->assertEquals(['module' => $this->root . '/place'], $module_handler->getModuleDirectories());
+    $module_handler->addModule('node', 'core/modules/node');
+    $this->assertEquals(['node' => $this->root . '/core/modules/node'], $module_handler->getModuleDirectories());
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
index 9c5fd58fadc0..03f2af113f61 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
@@ -31,29 +31,30 @@ class ThemeExtensionListTest extends UnitTestCase {
     $extension_discovery
       ->scan('theme')
       ->willReturn([
-        'test_subtheme'  => new Extension($this->root, 'theme', $this->root . '/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml', 'test_subtheme.info.yml'),
-        'test_basetheme' => new Extension($this->root, 'theme', $this->root . '/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml', 'test_basetheme.info.yml'),
+        'test_subtheme'  => new Extension($this->root, 'theme', 'core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml', 'test_subtheme.info.yml'),
+        'test_basetheme' => new Extension($this->root, 'theme', 'core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml', 'test_basetheme.info.yml'),
       ]);
     $extension_discovery
       ->scan('theme_engine')
       ->willReturn([
-        'twig' => new Extension($this->root, 'theme_engine', $this->root . '/core/themes/engines/twig/twig.info.yml', 'twig.engine'),
+        'twig' => new Extension($this->root, 'theme_engine', 'core/themes/engines/twig/twig.info.yml', 'twig.engine'),
       ]);
 
     // Verify that info parser is called with the specified paths.
     $argument_condition = function ($path) {
       return in_array($path, [
-        $this->root . '/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml',
-        $this->root . '/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml',
-        $this->root . '/core/themes/engines/twig/twig.info.yml',
+        'core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml',
+        'core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml',
+        'core/themes/engines/twig/twig.info.yml',
       ], TRUE);
     };
     $info_parser = $this->prophesize(InfoParserInterface::class);
+    $root = $this->root;
     $info_parser->parse(Argument::that($argument_condition))
       ->shouldBeCalled()
-      ->will(function ($file) {
+      ->will(function ($file) use ($root) {
         $info_parser = new InfoParser();
-        return $info_parser->parse($file[0]);
+        return $info_parser->parse($root . '/' . $file[0]);
       });
 
     $module_handler = $this->prophesize(ModuleHandlerInterface::class);
@@ -99,9 +100,9 @@ class ThemeExtensionListTest extends UnitTestCase {
     $info_subtheme->info['base theme'] = 'test_basetheme';
     $info_basetheme->sub_themes = ['test_subtheme'];
 
-    $this->assertEquals($this->root . '/core/themes/engines/twig/twig.engine', $info_basetheme->owner);
+    $this->assertEquals('core/themes/engines/twig/twig.engine', $info_basetheme->owner);
     $this->assertEquals('twig', $info_basetheme->prefix);
-    $this->assertEquals($this->root . '/core/themes/engines/twig/twig.engine', $info_subtheme->owner);
+    $this->assertEquals('core/themes/engines/twig/twig.engine', $info_subtheme->owner);
     $this->assertEquals('twig', $info_subtheme->prefix);
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
index c0883f1548ee..99c676947797 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -80,7 +80,7 @@ class ThemeHandlerTest extends UnitTestCase {
     $this->themeList->expects($this->at(1))
       ->method('getList')
       ->will($this->returnValue([
-        'seven' => new Extension($this->root, 'theme', $this->root . '/core/themes/seven/seven.info.yml', 'seven.theme'),
+        'seven' => new Extension($this->root, 'theme', 'core/themes/seven/seven.info.yml', 'seven.theme'),
       ]));
 
     $theme_data = $this->themeHandler->rebuildThemeData();
@@ -90,8 +90,8 @@ class ThemeHandlerTest extends UnitTestCase {
     // Ensure some basic properties.
     $this->assertInstanceOf('Drupal\Core\Extension\Extension', $info);
     $this->assertEquals('seven', $info->getName());
-    $this->assertEquals($this->root . '/core/themes/seven/seven.info.yml', $info->getPathname());
-    $this->assertEquals($this->root . '/core/themes/seven/seven.theme', $info->getExtensionPathname());
+    $this->assertEquals('core/themes/seven/seven.info.yml', $info->getPathname());
+    $this->assertEquals('core/themes/seven/seven.theme', $info->getExtensionPathname());
 
   }
 
@@ -99,7 +99,7 @@ class ThemeHandlerTest extends UnitTestCase {
    * Tests empty libraries in theme.info.yml file.
    */
   public function testThemeLibrariesEmpty() {
-    $theme = new Extension($this->root, 'theme', '/core/modules/system/tests/themes/test_theme_libraries_empty', 'test_theme_libraries_empty.info.yml');
+    $theme = new Extension($this->root, 'theme', 'core/modules/system/tests/themes/test_theme_libraries_empty', 'test_theme_libraries_empty.info.yml');
     try {
       $this->themeHandler->addTheme($theme);
       $this->assertTrue(TRUE, 'Empty libraries key in theme.info.yml does not cause PHP warning');
diff --git a/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php b/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
index a6717326b9cb..3de9078e918d 100644
--- a/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
@@ -70,7 +70,7 @@ class LayoutPluginManagerTest extends UnitTestCase {
     $this->moduleHandler->moduleExists('core')->willReturn(FALSE);
     $this->moduleHandler->moduleExists('invalid_provider')->willReturn(FALSE);
 
-    $module_a = new Extension('/', 'module', vfsStream::url('root/modules/module_a/module_a.layouts.yml'));
+    $module_a = new Extension('vfs://root', 'module', 'modules/module_a/module_a.layouts.yml');
     $this->moduleHandler->getModule('module_a')->willReturn($module_a);
     $this->moduleHandler->getModuleDirectories()->willReturn(['module_a' => vfsStream::url('root/modules/module_a')]);
     $this->moduleHandler->alter('layout', Argument::type('array'))->shouldBeCalled();
@@ -81,7 +81,7 @@ class LayoutPluginManagerTest extends UnitTestCase {
     $this->themeHandler->themeExists('core')->willReturn(FALSE);
     $this->themeHandler->themeExists('invalid_provider')->willReturn(FALSE);
 
-    $theme_a = new Extension('/', 'theme', vfsStream::url('root/themes/theme_a/theme_a.layouts.yml'));
+    $theme_a = new Extension('vfs://root', 'theme', 'themes/theme_a/theme_a.layouts.yml');
     $this->themeHandler->getTheme('theme_a')->willReturn($theme_a);
     $this->themeHandler->getThemeDirectories()->willReturn(['theme_a' => vfsStream::url('root/themes/theme_a')]);
 
@@ -112,7 +112,6 @@ class LayoutPluginManagerTest extends UnitTestCase {
    * @covers ::processDefinition
    */
   public function testGetDefinition() {
-    $theme_a_path = vfsStream::url('root/themes/theme_a');
     $layout_definition = $this->layoutPluginManager->getDefinition('theme_a_provided_layout');
     $this->assertSame('theme_a_provided_layout', $layout_definition->id());
     $this->assertSame('2 column layout', (string) $layout_definition->getLabel());
@@ -122,10 +121,10 @@ class LayoutPluginManagerTest extends UnitTestCase {
     $this->assertTrue($layout_definition->getCategory() instanceof TranslatableMarkup);
     $this->assertTrue($layout_definition->getDescription() instanceof TranslatableMarkup);
     $this->assertSame('twocol', $layout_definition->getTemplate());
-    $this->assertSame("$theme_a_path/templates", $layout_definition->getPath());
+    $this->assertSame('themes/theme_a/templates', $layout_definition->getPath());
     $this->assertSame('theme_a/twocol', $layout_definition->getLibrary());
     $this->assertSame('twocol', $layout_definition->getThemeHook());
-    $this->assertSame("$theme_a_path/templates", $layout_definition->getTemplatePath());
+    $this->assertSame('themes/theme_a/templates', $layout_definition->getTemplatePath());
     $this->assertSame('theme_a', $layout_definition->getProvider());
     $this->assertSame('right', $layout_definition->getDefaultRegion());
     $this->assertSame(LayoutDefault::class, $layout_definition->getClass());
@@ -142,7 +141,6 @@ class LayoutPluginManagerTest extends UnitTestCase {
     $this->assertTrue($regions['left']['label'] instanceof TranslatableMarkup);
     $this->assertTrue($regions['right']['label'] instanceof TranslatableMarkup);
 
-    $module_a_path = vfsStream::url('root/modules/module_a');
     $layout_definition = $this->layoutPluginManager->getDefinition('module_a_provided_layout');
     $this->assertSame('module_a_provided_layout', $layout_definition->id());
     $this->assertSame('1 column layout', (string) $layout_definition->getLabel());
@@ -152,7 +150,7 @@ class LayoutPluginManagerTest extends UnitTestCase {
     $this->assertTrue($layout_definition->getCategory() instanceof TranslatableMarkup);
     $this->assertTrue($layout_definition->getDescription() instanceof TranslatableMarkup);
     $this->assertSame(NULL, $layout_definition->getTemplate());
-    $this->assertSame("$module_a_path/layouts", $layout_definition->getPath());
+    $this->assertSame('modules/module_a/layouts', $layout_definition->getPath());
     $this->assertSame('module_a/onecol', $layout_definition->getLibrary());
     $this->assertSame('onecol', $layout_definition->getThemeHook());
     $this->assertSame(NULL, $layout_definition->getTemplatePath());
@@ -226,7 +224,6 @@ EOS;
    */
   public function testGetThemeImplementations() {
     $core_path = '/core/lib/Drupal/Core';
-    $theme_a_path = vfsStream::url('root/themes/theme_a');
     $expected = [
       'layout' => [
         'render element' => 'content',
@@ -235,7 +232,7 @@ EOS;
         'render element' => 'content',
         'base hook' => 'layout',
         'template' => 'twocol',
-        'path' => "$theme_a_path/templates",
+        'path' => 'themes/theme_a/templates',
       ],
       'plugin_provided_layout' => [
         'render element' => 'content',
diff --git a/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
index 65266c9d8af8..7ecd60e59cae 100644
--- a/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
@@ -326,6 +326,7 @@ EOF;
     vfsStream::create([
       'modules' => [
         'test_module' => [
+          'test_module.info.yml' => $test_module_info,
           'tests' => [
             'src' => [
               'Functional' => [
@@ -491,7 +492,7 @@ EOF;
 
     $test_discovery = new TestDiscovery('vfs://drupal', $class_loader->reveal(), $module_handler->reveal());
 
-    $result = $test_discovery->getTestClasses(NULL, ['PHPUnit-Kernel']);
+    $result = $test_discovery->getTestClasses('test_profile_module', ['PHPUnit-Kernel']);
     $expected = [
       'example3' => [
         'Drupal\Tests\test_profile_module\Kernel\KernelExampleTest4' => [