Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/modules/admin_toolbar/tests/src/Functional/AdminToolbarAlterTest.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/admin_toolbar/tests/src/Functional/AdminToolbarAlterTest.php on line 12