Fatal error: Uncaught Error: Class "Drupal\Tests\system\Functional\Module\ModuleTestBase" not found in /home/www/drupal8/test/core/modules/config/tests/src/Functional/ConfigImportAllTest.php:15
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/config/tests/src/Functional/ConfigImportAllTest.php on line 15