Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/core/modules/file/tests/src/Kernel/ComputedFileUrlTest.php:16 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/file/tests/src/Kernel/ComputedFileUrlTest.php on line 16