About 5 or 6 years ago we had an alert in the middle of the night that our RDS instance dieded. It failed over in about 15 seconds (SQL Server so it’s a bit slow compared to PostgreSQL) but the MultiAZ worked as advertised. The downside is AWS never told us why it occurred.
I’ve seen a few AWS instance hardware failures, they happen with some regularity.
You can handle single instance failure without being multi AZ.
Testing an actual AZ failure, as in the whole AZ going offline or getting partitioned from the other AZs, is pretty much impossible.
Those are basic (don't cover flapping or glacial-speed slowdown degradation modes, some services only, etc) but a starting point at least that can be extended.
