Improve the Content API test helpers (again)

There's a few improvements to the Content API test helpers rolled up into this pull request:

- I've fixed a typo which broke one of the test helpers.
- I've changed the existing Content API adapter tests so that all the tag-related tests use the test helpers to stub requests instead of using Webmock, so we can actually catch problems with the test helpers.
- I've fixed an issue with `content_api_has_artefacts_with_a_tag` where the stubbed requests relied on the order of the query string to work properly. Previously, this wouldn't stub the right URLs if the tag type was before 'sort' in the alphabet.
- I've removed the use of the `include_children` query string parameter (which was removed from the Content API in alphagov/govuk_content_api#98).
- I've added a new test helper for stubbing a request to `with_tag.json` with a custom sort order.