Refactors rack attack requests spec

What does this MR do?

On https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31246/diffs, we're moving Protected Paths from Omnibus into GitLab-Rails. This implied some modifications to spec/requests/rack_attack_global_spec.rb to also cover the new throttles.

Since that MR is getting larger. I've decided to ship the refactor into a different MR to facilitate the review.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers