irmin-tezos (test_gc): Add `check_async_unlinked` that checks if a file has been unlinked asynchronously.

[adatario]

This fixes flakey test failures introduced by #2221 caused by files still existing that are being unlinked asynchronously (see https://github.com/mirage/irmin/pull/2227#issuecomment-1486707570).

The introduced check_asyn_unlinked function takes a timeout argument and checks if the file has been unlinked within the specified timeout period.

Tested locally:

• [x] Use negative timeout to check failure
• [ ] Use small timeout to reproduce condition that should be catched.

I've run the tests multiple times and can not reproduce the conditions observed in the CI. Still, I believe this should fix the issue.