WIP: Add instance file template tests - Blocked

What does this MR do?

Adds tests for instance-wide file templates.

gitlab-org/quality/testcases#113

Currently blocked by #36663 details here gitlab-org/quality/testcases#113 (comment 234004064)

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