Add guideline to use example urls

Add guideling to use example urls in rspec.

doc/development/testing_guide/best_practices.md

@@ -59,6 +59,7 @@ When using spring and guard together, use `SPRING=1 bundle exec guard` instead t
 - Try to follow the [Four-Phase Test](https://thoughtbot.com/blog/four-phase-test) pattern, using newlines
   to separate phases.
 - Use `Gitlab.config.gitlab.host` rather than hard coding `'localhost'`.
+- For literal URLs in tests, use `example.com`, `gitlab.example.com`. This will ensure that we do not utilize any real URLs.
 - Don't assert against the absolute value of a sequence-generated attribute (see
   [Gotchas](../gotchas.md#do-not-assert-against-the-absolute-value-of-a-sequence-generated-attribute)).
 - Avoid using `expect_any_instance_of` or `allow_any_instance_of` (see