Fatal error: Uncaught Error: Class "Drupal\simpletest\WebTestBase" not found in /home/www/drupal8/test/core/modules/system/src/Tests/Database/DatabaseWebTestBase.php:16 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/system/src/Tests/Database/DatabaseWebTestBase.php on line 16