Replace the 'project/merge_requests.feature' spinach test with an rspec analog

What does this MR do?

Replaces the EE related spinach specs as part of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14621

Why was this MR needed?

This is a part of the spinach removal campaign.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14621