test_room_keys_received_on_notification_client_trigger_redecryption test is flaky

[richvdh]

This test was introduced in 0ca35d6 (#4348), but it looks like it's flaky: https://github.com/matrix-org/matrix-rust-sdk/actions/runs/12463937263/job/34787181014

It looks like Alice didn't actually send the to-device message to Bob. Presumably she hadn't yet received the memo that he had joined.

[bnjbvr]

Another failure, apparently a different failure mode: the notification client fails to decrypt the event:

https://github.com/matrix-org/matrix-rust-sdk/actions/runs/12632299265/job/35195713771

[bnjbvr]

Another instance here: https://github.com/matrix-org/matrix-rust-sdk/actions/runs/12634558366/job/35202551369.
Same line and diagnostic as my previous comment.

[poljar]

This should be fixed by #4468.

[bnjbvr]

Thank you Damir!