TUN-7480: Added a timeout for unregisterUDP.

I deliberately kept this as an unregistertimeout because that was the
intent. In the future we could change this to a UDPConnConfig if we want
to pass multiple values here.

The idea of this PR is simply to add a configurable unregister UDP
timeout.

connection/quic_test.go

@@ -725,6 +725,7 @@ func testQUICConnection(udpListenerAddr net.Addr, t *testing.T, index uint8) *QU
 		fakeControlStream{},
 		&log,
 		nil,
+		5*time.Second,
 	)
 	require.NoError(t, err)
 	return qc