Add fsyncUnlock command tests

[danielfrankcom]

This change adds tests for the fsyncUnlock administration command.

This command affects global state, so it uses the no_parallel mark, and also has an autouse fixture which makes sure we have a clean state before running each test.

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-framework · Priority P2 · Effort M · Status Needs Review
Confidence: 0.88 (mixed)

Reasoning

component from path globs (test-framework, test-coverage); effort from diff stats (403+2 LOC, 3 files); LLM: Adds new test coverage for the fsyncUnlock command in the compatibility test suite, a straightforward test addition.

If a label is wrong, remove it manually and ping @patty-chow so the rules can be tuned. The bot will not re-label items that already have component labels.