Add an acceptance test for the logic provided by `service:head-data`

PR https://github.com/ember-learn/guides-source/pull/1980 on ember-learn/guides-source removes a unit test on `service:head-data`

This service is not provided by `ember-learn/guides-source`, but rather by `empress/guidemaker` and as such, should be tested here.

It should be tested with an acceptance test though, not a unit test, and the logic in test https://github.com/empress/empress-blog/blob/master/tests/acceptance/meta-test.js could should probably be replicated.

This issue is here to remind us that this test should be added at some point, since this service won't be tested anymore once https://github.com/ember-learn/guides-source/pull/1980 gets accepted.

Start by reading the referenced empress-blog test at tests/acceptance/meta-test.js and locating the acceptance-test setup in guidemaker. Add coverage for the service:head-data logic as an acceptance test, and confirm the test passes and covers the behavior that the guides-source unit test removal leaves untested.

Written by the indexing model from the issue text.