Extend content api test helpers to support options

* Extend `#content_api_has_artefacts_with_a_tag` to take a options hash.
This allows the interface to be flexible enough for a wider range of
testing situations, i.e. artefacts in a specific format.
Also, if this is correct, I think `options.fetch(:format, "Guide")` does the same thing, and is easier to read.