message: integration_tests: use var instead of define
  This allows to test with many mission items but instead of a define we
  use a variable, so that we can catch compile errors.
diff: @@ -26,8 +26,8 @@ static std::shared_ptr<MissionItem> add_mission_item(double latitude_deg,
  static void compare_mission_items(const std::shared_ptr<MissionItem> original,
  const std::shared_ptr<MissionItem> downloaded);
  -// Set to 1 to test with 1200 mission items.
  -#define MANY_ITEMS_TEST 0
  +// Set to 50 to test with about 1200 mission items.
  +static constexpr int test_with_many_items = 1;
  TEST_F(SitlTest, MissionAddWaypointsAndFly)
  @@ -66,9 +66,7 @@ TEST_F(SitlTest, MissionAddWaypointsAndFly)
  std::vector<std::shared_ptr<MissionItem>> mission_items;
  -#if MANY_ITEMS_TEST==1
  - for (int i = 0; i < 50; ++i) {
  -#endif
  + for (int i = 0; i < test_with_many_items; ++i) {
  mission_items.push_back(
  add_mission_item(47.398170327054473,
  8.5456490218639658,
  @@ -116,9 +114,7 @@ TEST_F(SitlTest, MissionAddWaypointsAndFly)
  NAN,
  MissionItem::CameraAction::STOP_PHOTO_INTERVAL));
  -#if MANY_ITEMS_TEST==1
  }
  -#endif
  {
  LogInfo() << "Uploading mission...";
  