Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php:23 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php on line 23