Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php on line 15