Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/core/modules/serialization/tests/src/Kernel/NormalizerTestBase.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/serialization/tests/src/Kernel/NormalizerTestBase.php on line 12