Quarantine two snippet tests due to a bug

What does this MR do?

There is a Gitaly issue that causes intermittent failure on Staging for two snippet tests. See #239091 (closed) and gitaly#3143 (closed) for more details.

Though the problem was identified, it will likely take some time to find a solution, so we decided to quarantine the tests until then to reduce the noise for the pipeline triage.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content