Some roster-based tests depend on there not being any prior subscription state beteween entities. The utility method that
tries to guarantee that, acts on the state of the roster that's cached in memory, but acts on the one that's stored on the
server. This occasionally causes issues, as both representations might be different.
Stability is added in this commit by:
- refreshing the roster from the server prior to evaluating it
- ignoring an 'item-not-found' as returned by the server, when the code tries to remove that item.
The pattern
if (now > deadline) break;
wait(deadline - now);
is insufficient in case "now == deadline" because the result would be
wait() being called with 0, which would mean "wait until
notified". Thus, the timeout would become infinite.
