message: integration_tests: example impl for systems()
  This is a try to use the new systems() method.
diff: @@ -38,14 +38,21 @@ void takeoff_and_hover_at_altitude(float altitude_m)
  ASSERT_TRUE(poll_condition_with_timeout(
  [&mavsdk]() { return mavsdk.is_connected(); }, std::chrono::seconds(10)));
  - System& system = mavsdk.system();
  - auto telemetry = std::make_shared<Telemetry>(system);
  + auto systems = dc.systems();
  +
  + ASSERT_EQ(systems.size(), 1);
  +
  + auto system = systems.at(0);
  +
  + auto telemetry = std::make_shared<Telemetry>(*system.lock());
  LogInfo() << "Waiting for system to be ready";
  ASSERT_TRUE(poll_condition_with_timeout(
  [telemetry]() { return telemetry->health_all_ok(); }, std::chrono::seconds(10)));
  - auto action = std::make_shared<Action>(system);
  + auto action = std::make_shared<Action>(*system.lock());
  + Action::Result action_ret = action->arm();
  + EXPECT_EQ(action_ret, Action::Result::SUCCESS);
  EXPECT_EQ(Action::Result::Success, action->set_takeoff_altitude(altitude_m));
  auto takeoff_altitude_result = action->get_takeoff_altitude();
  