Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/modules/contrib/swiftmailer/tests/src/Kernel/Utility/ConversionTest.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/swiftmailer/tests/src/Kernel/Utility/ConversionTest.php on line 12