Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/modules/contrib/swiftmailer/tests/src/Kernel/Plugin/Mail/SwiftMailerTest.php:13 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/swiftmailer/tests/src/Kernel/Plugin/Mail/SwiftMailerTest.php on line 13