Fatal error: Uncaught Error: Class "Drupal\Tests\serialization\Kernel\NormalizerTestBase" not found in /home/www/drupal8/test/core/modules/serialization/tests/src/Kernel/FieldItemSerializationTest.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/serialization/tests/src/Kernel/FieldItemSerializationTest.php on line 15