Fatal error: Uncaught Error: Class "Drupal\Tests\field\Kernel\FieldKernelTestBase" not found in /home/www/drupal8/test/core/modules/link/tests/src/Kernel/LinkItemSerializationTest.php:17 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/link/tests/src/Kernel/LinkItemSerializationTest.php on line 17