Handle transactions sent during UnverifiableEvents test (#59)

Once again Synapse likes to send presence when it joins a room. I wonder if we
should perhaps make federation.HandleTransactionRequests default for new
servers, with the ability to override it if desired?

The only thing I worry about there is that you may want a test that 404's on
transactions.  Maybe at that point we could just extend
HandleTransactionRequests to return custom responses.

tests/federation_room_join_test.go

@@ -98,6 +98,7 @@ func TestJoinFederatedRoomWithUnverifiableEvents(t *testing.T) {
 	srv := federation.NewServer(t, deployment,
 		federation.HandleKeyRequests(),
 		federation.HandleMakeSendJoinRequests(),
+		federation.HandleTransactionRequests(nil, nil),
 	)
 	srv.UnexpectedRequestsAreErrors = false
 	cancel := srv.Listen()