Depending on timing, the first `httpBackend.flush()` could end up just flushing
a '/presence' call rather than the initial sync. The fix to that is simply to
not set the expectation on /presence.
While we're there, split out the flushes of /publicRooms and
/thirdparty/protocols, so that we can be sure that they happen.
There is probably still a bunch of flakiness there, but this should fix one
particular instance.
593e101521