Add high level Engine tests

Quoting from: https://gitlab.com/gitlab-org/gitlab-triage/merge_requests/48#note_93756560

We didn't have high level tests for Engine. Remove all the changes in Engine all tests would still pass, but we're not supporting this feature at all. This came to my mind before but I didn't bother to work on either... I'll create an issue.

We need to have tests which would really hit code paths using all the filters and limiters to make sure they're really working in a high level sense.