Flake: If a device list update goes missing, the server resyncs on the next one

https://github.com/matrix-org/synapse/actions/runs/5699129687/job/15447723403

Start with the linked GitHub Actions job and inspect its failure output and surrounding sytest context. The issue body names no test file or entry point, so first identify the failing device-list update scenario; done should mean the missing update no longer produces a flake and the relevant test passes reliably.

Written by the indexing model from the issue text.