Add restricted user/group protected branch test

What does this MR do?

Adds an E2E test of protected branches with access granted to specific users/groups (not just roles).

Closes gitlab-org/quality/testcases#117 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] 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