Fatal error: Uncaught Error: Class "Drupal\KernelTests\Core\Entity\EntityKernelTestBase" not found in /home/www/drupal8/test/core/modules/user/tests/src/Kernel/UserEntityReferenceTest.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/user/tests/src/Kernel/UserEntityReferenceTest.php on line 15