Quarantine failing "rebase_merge_request_spec.rb"

What does this MR do?

Quarantines failing "rebase_merge_request_spec.rb"

Related to: gitlab-org/quality/staging#66 (moved)

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