Update to Drupal 8.0.2. For more information, see https://www.drupal.org/drupal-8.0.2-release-notes

core/modules/simpletest/src/WebTestBase.php

@@ -1010,6 +1010,12 @@ abstract class WebTestBase extends TestBase {
   /**
    * Install modules defined by `static::$modules`.
    *
+   * To install test modules outside of the testing environment, add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @encode
+   * to your settings.php.
+   *
    * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
    *   The container.
    */