Add test_combined_retention_policies to verify all 5 retention policies
(keep_all, keep_daily, keep_weekly, keep_monthly, keep_yearly) work
correctly together. Test validates precedence rules where later retention
tiers can override earlier ones (e.g., monthly overrides weekly, yearly
overrides monthly) and documents threshold inclusive behavior.
Test uses 36 backups spanning 2017-2021 with expected 21 backups kept
based on policy interaction.
Maks Snegov
06583f7e1a