Add tests for tier store backup retry semantics

Refactor tier_store unit tests around shared filesystem-backed test
fixtures and add failure-injection helpers for backup and retry stores.

Cover best-effort backup failure handling, semisync retry enqueueing,
queue-persist failure behavior, retry-task replay of queued writes and
removes, remove-not-found idempotence, retry-queue restart reload,
dedup behavior across replacement sequences, stale snapshot skipping,
and pending-op serialization round-trips.

Author: enigbe