Fix a race condition in the dns/bufferevent_connect_hostname test.

Fix a race condition in the dns/bufferevent_connect_hostname test.

As originally written, the test would only pass if the accept()
callbacks for the evconnlistener were all invoked before the last of
the CONNECTED/ERROR callbacks for the connecting/resolving bufferevent
had its call to event_base_loopexit() complete.  But this was only
accidentally true in 2.0, and might not be true at all in 2.1 where
we schedule event_base_once() callbacks more aggressively.

Found by Sebastian Hahn.