message: print_mode -> observe_mode because print suggests no side effects
diff: using namespace mavsdk;
  -void print_mode(Telemetry::FlightMode flight_mode);
  +void observe_mode(Telemetry::FlightMode flight_mode);
  static Telemetry::FlightMode _flight_mode = Telemetry::FlightMode::Unknown;
  TEST_F(SitlTest, TelemetryFlightModes)
  @@ -22,7 +22,7 @@ TEST_F(SitlTest, TelemetryFlightModes)
  auto telemetry = std::make_shared<Telemetry>(system);
  auto action = std::make_shared<Action>(system);
  - telemetry->flight_mode_async(std::bind(&print_mode, std::placeholders::_1));
  + telemetry->flight_mode_async(std::bind(&observe_mode, std::placeholders::_1));
  while (!telemetry->health_all_ok()) {
  std::cout << "waiting for system to be ready" << std::endl;
  @@ -39,7 +39,7 @@ TEST_F(SitlTest, TelemetryFlightModes)
  ASSERT_EQ(_flight_mode, Telemetry::FlightMode::Land);
  }
  -void print_mode(Telemetry::FlightMode flight_mode)
  +void observe_mode(Telemetry::FlightMode flight_mode)
  {
  std::cout << "Got FlightMode: " << flight_mode << std::endl;
  _flight_mode = flight_mode;
  