unit test coverage

[raulk]

Some missing unit tests as of now.

DAG store

• [ ] Test slow registration with acquirers queued up.
• [ ] Test slow registration without acquirers queued up.
• [ ] Test slow registration + getting shard state in the background.
• [ ] Test indexing failure on corrupted mount, no queued acquirers.
• [ ] Test queued acquirers fail on indexing failure.

Mount upgrader

• [x] Test deleted transient refetches.
• [x] Test refetch deduplication.