Fatal error: Uncaught Error: Class "Drupal\KernelTests\Core\File\FileTestBase" not found in /home/www/drupal8/test/core/modules/migrate/tests/src/Kernel/process/DownloadTest.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/migrate/tests/src/Kernel/process/DownloadTest.php on line 18