message: integration_tests: check systems before accessing
  This makes the tests fail a bit more gracefully if no system is
  connected.
diff: @@ -31,6 +31,7 @@ TEST(SitlTestGimbal, GimbalMove)
  // Wait for system to connect via heartbeat.
  std::this_thread::sleep_for(std::chrono::seconds(2));
  + ASSERT_EQ(mavsdk.systems().size(), 1);
  auto system = mavsdk.systems().at(0);
  // FIXME: This is what it should be, for now though with the typhoon_h480
  @@ -59,6 +60,7 @@ TEST(SitlTestGimbal, GimbalAngles)
  // Wait for system to connect via heartbeat.
  std::this_thread::sleep_for(std::chrono::seconds(2));
  + ASSERT_EQ(mavsdk.systems().size(), 1);
  auto system = mavsdk.systems().at(0);
  // FIXME: This is what it should be, for now though with the typhoon_h480
  @@ -106,6 +108,7 @@ TEST(SitlTestGimbal, GimbalTakeoffAndMove)
  // Wait for system to connect via heartbeat.
  std::this_thread::sleep_for(std::chrono::seconds(2));
  + ASSERT_EQ(mavsdk.systems().size(), 1);
  auto system = mavsdk.systems().at(0);
  // ASSERT_TRUE(system.has_gimbal());
  ASSERT_TRUE(system->has_autopilot());
  @@ -214,6 +217,7 @@ TEST(SitlTestGimbal, GimbalROIOffboard)
  // Wait for system to connect via heartbeat.
  std::this_thread::sleep_for(std::chrono::seconds(2));
  + ASSERT_EQ(mavsdk.systems().size(), 1);
  auto system = mavsdk.systems().at(0);
  // ASSERT_TRUE(system.has_gimbal());
  ASSERT_TRUE(system->has_autopilot());
  