Issue #3291830 by webflo: Fix PSR4 path for database driver

(cherry picked from commit a80dddcc3a)

core/lib/Drupal/Core/Site/Settings.php

@@ -190,7 +190,7 @@ final class Settings {
         // the database. Therefore, allow the connection info to specify an
         // autoload directory for the driver.
         if (isset($info['autoload'])) {
-          $class_loader->addPsr4($info['namespace'] . '\\', $info['autoload']);
+          $class_loader->addPsr4($info['namespace'] . '\\', $app_root . '/' . $info['autoload']);
         }
       }
     }

core/tests/Drupal/Tests/Core/Site/SettingsTest.php

@@ -336,7 +336,7 @@ class SettingsTest extends UnitTestCase {
     if (!empty($expected_autoload)) {
       $class_loader->expects($this->once())
         ->method('addPsr4')
-        ->with($expected_namespace . '\\', $expected_autoload);
+        ->with($expected_namespace . '\\', vfsStream::url('root') . '/' . $expected_autoload);
     }
     else {
       $class_loader->expects($this->never())