Fix flaky spec for merge request lists

What does this MR do?

Fixes the spec that assumed that the first element was the merge request for which it was searching. It now selects by id instead, and makes sure it has the correct title.

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes !48712- Flaky spec / red master: API::MergeRequests GET /groups/:id/merge_requests behaves like merge requests list when authenticated returns an array of merge requests in given milestone