Fatal error: Uncaught Error: Class "Drupal\Tests\UnitTestCase" not found in /home/www/drupal8/test/modules/contrib/mailsystem/tests/src/Unit/MailsystemManagerTest.php:20 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/mailsystem/tests/src/Unit/MailsystemManagerTest.php on line 20