Fatal error: Uncaught Error: Class "Drupal\Tests\file\Kernel\FileManagedUnitTestBase" not found in /home/www/drupal8/test/core/modules/file/tests/src/Kernel/ValidateTest.php:10 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/file/tests/src/Kernel/ValidateTest.php on line 10