diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index 6ca7e89f803..f7c97fc873e 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<phpunit bootstrap="tests/bootstrap.php" colors="true">
+<phpunit bootstrap="tests/bootstrap.php" colors="true" strict="true">
   <testsuites>
     <testsuite name="Drupal Unit Test Suite">
       <directory>./tests/*</directory>
diff --git a/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageTest.php b/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageTest.php
index 3cd1e624691..55d01c8ee09 100644
--- a/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageTest.php
+++ b/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageTest.php
@@ -57,6 +57,9 @@ class MTimeProtectedFileStorageTest extends PhpStorageTestBase {
    *
    * We test two attacks: first changes the file mtime, then the directory
    * mtime too.
+   *
+   * We need to delay over 1 second for mtime test.
+   * @medium
    */
   function testSecurity() {
     $php = $this->storageFactory->get('simpletest');
diff --git a/core/tests/Drupal/Tests/Core/Asset/CssCollectionRendererUnitTest.php b/core/tests/Drupal/Tests/Core/Asset/CssCollectionRendererUnitTest.php
index d8e3a329475..bfbe65c2635 100644
--- a/core/tests/Drupal/Tests/Core/Asset/CssCollectionRendererUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/CssCollectionRendererUnitTest.php
@@ -153,7 +153,7 @@ class CssCollectionRendererUnitTest extends UnitTestCase {
    *
    * @see testRender
    */
-  function testRenderProvider() {
+  function providerTestRender() {
     // Default for 'browsers' key in CSS asset.
     $browsers_default = array('IE' => TRUE, '!IE' => TRUE);
 
@@ -538,7 +538,7 @@ class CssCollectionRendererUnitTest extends UnitTestCase {
   /**
    * Tests CSS asset rendering.
    *
-   * @dataProvider testRenderProvider
+   * @dataProvider providerTestRender
    */
   function testRender(array $css_assets, array $render_elements) {
     $this->state->expects($this->once())
diff --git a/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php b/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
index a077ad53653..2fbf61952de 100644
--- a/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
@@ -86,7 +86,7 @@ class CssOptimizerUnitTest extends UnitTestCase {
   /**
    * Provides data for the CSS asset optimizing test.
    */
-  function testOptimizeProvider() {
+  function providerTestOptimize() {
     $path = dirname(__FILE__)  . '/css_test_files/';
     return array(
       // File. Tests:
@@ -194,7 +194,7 @@ class CssOptimizerUnitTest extends UnitTestCase {
   /**
    * Tests optimizing a CSS asset group containing 'type' => 'file'.
    *
-   * @dataProvider testOptimizeProvider
+   * @dataProvider providerTestOptimize
    */
   function testOptimize($css_asset, $expected) {
     $this->assertEquals($expected, $this->optimizer->optimize($css_asset), 'Group of file CSS assets optimized correctly.');
diff --git a/core/tests/Drupal/Tests/Core/Database/EmptyStatementTest.php b/core/tests/Drupal/Tests/Core/Database/EmptyStatementTest.php
index 9bf2c8071e3..6877a9ce35b 100644
--- a/core/tests/Drupal/Tests/Core/Database/EmptyStatementTest.php
+++ b/core/tests/Drupal/Tests/Core/Database/EmptyStatementTest.php
@@ -39,10 +39,11 @@ class EmptyStatementTest extends UnitTestCase {
   function testEmptyIteration() {
     $result = new StatementEmpty();
 
+    $count = 0;
     foreach ($result as $record) {
-      $this->fail('Iterating empty result set should not iterate.');
-      return;
+      $count++;
     }
+    $this->assertSame(0, $count, 'Iterating empty result set should not iterate.');
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php
index 45ad0d26369..49ab1f83c9b 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php
@@ -13,6 +13,7 @@ if (!defined('DRUPAL_ROOT')) {
 
 use Drupal\Core\Extension\ModuleHandler;
 use Drupal\Tests\UnitTestCase;
+use PHPUnit_Framework_Error_Notice;
 
 /**
  * Tests the ModuleHandler class.
@@ -34,9 +35,12 @@ class ModuleHandlerUnitTest extends UnitTestCase {
     $this->moduleHandler = new ModuleHandler;
   }
 
-  function testloadInclude() {
-    // Make sure that load include does not throw notices on nonexisiting
-    // modules.
-    $this->moduleHandler->loadInclude('foo', 'inc');
+  /**
+   * Tests loading of an include from a nonexistent module.
+   */
+  public function testLoadInclude() {
+    // Attepmting to load a file from a non-existent module should return FALSE.
+    $this->assertFalse($this->moduleHandler->loadInclude('foo', 'inc'));
   }
+
 }
diff --git a/core/tests/Drupal/Tests/Core/Lock/LockBackendAbstractTest.php b/core/tests/Drupal/Tests/Core/Lock/LockBackendAbstractTest.php
index 933f8b5fbfa..2ce4ef7fe01 100644
--- a/core/tests/Drupal/Tests/Core/Lock/LockBackendAbstractTest.php
+++ b/core/tests/Drupal/Tests/Core/Lock/LockBackendAbstractTest.php
@@ -52,6 +52,9 @@ class LockBackendAbstractTest extends UnitTestCase {
 
   /**
    * Tests the wait() method when lockMayBeAvailable() returns FALSE.
+   *
+   * Waiting could take 1 second so we need to extend the possible runtime.
+   * @medium
    */
   public function testWaitTrue() {
     $this->lock->expects($this->any())