Complement `TestDeviceListUpdates/when_joining_a_room_with_a_remote_user` is flakey(?)

[DMRobertson]

https://github.com/matrix-org/synapse/actions/runs/3149623842/jobs/5121449618#step:5:8174

Happened in worker mode---would guess there's some race the test isn't expecting?

[squahtx]

related to https://github.com/matrix-org/synapse/issues/13783

[squahtx]

I reckon the tests are picking up the spurious device_lists.changed instead of the real one and advancing too early.