Fatal error: Uncaught Error: Class "Drupal\Tests\field\Kernel\FieldKernelTestBase" not found in /home/www/drupal8/test/core/modules/file/tests/src/Kernel/FileItemTest.php:20 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/file/tests/src/Kernel/FileItemTest.php on line 20