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.