VFS: add test for dehydration of files not yet synced

[TheOneRing]

Files that are not yet uploaded must never be dehydrated.
Add a unit test for this scenario.

[github-actions]

This issue was marked stale because it has been open for 30 days with no activity. Remove the stale label or comment or this will be closed in 7 days.

[github-actions]

The issue was marked as stale for 7 days and closed automatically.