Refactor and Add Specs to Test Inherited Permissions for Merge Requests

What does this MR do and why?

This MR adds specs based on the modified P1 test cases from the permissions wizard in https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1404+ in order to test inherited user permissions around merge requests.

Not every test case is added yet, as this is to serve more as a foundational MR that includes examples and refactoring for other SETs to contribute. Once merged, the issue will be updated to reflect what additional tests will be needed.

Related to https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1413+

Epic: https://gitlab.com/groups/gitlab-org/quality/-/epics/35+

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.