`Real non-joined user doesn't get events before room made world_readable` is racy

sometimes this just times out, particularly when run against a server in worker mode.

It seems like the /events request isn't returning with the new events.

Locate the test described by “Real non-joined user doesn't get events before room made world_readable” and trace its /events request, especially when run against a server in worker mode. Reproduce the timeout and identify the event-ordering or synchronization condition; done means the test reliably receives the new events without timing out.

Written by the indexing model from the issue text.