Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/modules/system/tests/src/Functional/Mail/MailTest.php:20 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/system/tests/src/Functional/Mail/MailTest.php on line 20