message: core: add test back in
  This makes the unit test fail. The next commit will fix it.
diff: @@ -614,6 +614,10 @@ TEST_F(MAVLinkMissionTransferTest, UploadMissionNacksAreHandled)
  });
  mmt.do_work();
  + EXPECT_CALL(mock_sender, send_message(Truly([&items](const mavlink_message_t& message) {
  + return is_the_same_mission_item_int(items[0], message);
  + })));
  +
  message_handler.process_message(make_mission_request_int(MAV_MISSION_TYPE_MISSION, 0));
  // Send nack now.
  