Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/modules/user/tests/src/Functional/UserRegistrationTest.php:17
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/user/tests/src/Functional/UserRegistrationTest.php on line 17