Fatal error: Uncaught Error: Class 'Drupal\Tests\BrowserTestBase' not found in /var/www/html/tmp/contrib/mailsystem/tests/src/Functional/MailsystemTestThemeTest.php:13 Stack trace: #0 {main} thrown in /var/www/html/tmp/contrib/mailsystem/tests/src/Functional/MailsystemTestThemeTest.php on line 13