header sitemap integration tests

Second take on the header integration tests (see original PR here: https://github.com/discourse/discourse/pull/1652).

I've incorporated changes made by Sam here: https://github.com/discourse/discourse/commit/3ce6a7be4a49403b39819c41875f88400279a58e (which have broken my original PR) and as discussed in original PR made tests robust by adding css classes allowing checking for various elements.