Fatal error: Uncaught Error: Class "Drupal\simpletest\WebTestBase" not found in /home/www/drupal8/test/modules/contrib/mailsystem/src/Tests/MailsystemTestThemeTest.php:16 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/mailsystem/src/Tests/MailsystemTestThemeTest.php on line 16