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