Fatal error: Uncaught Error: Class "Drupal\simpletest\WebTestBase" not found in /home/www/drupal8/test/modules/contrib/swiftmailer/src/Tests/SwiftMailerSettingsTest.php:12
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/modules/contrib/swiftmailer/src/Tests/SwiftMailerSettingsTest.php on line 12