Add test to ensure room history is always visible as we decrypt historical keys

[MadLittleMods]

Your use case

What would you like to do?

Add test to ensure room history is always visible as we decrypt historical keys. Specifically a test for the scenario fixed by matrix-org/matrix-react-sdk#8563

Why would you like to do it?

To ensure we don't regress that area again. The order of operations is pretty finicky there.

Have you considered any alternatives?

No response

Additional context

No response