message: mission: fix mission plan path for macOS
  As long as we run the unit test from the root of the Dronecode SDK, the
  path is fine hard-coded as is.
  This fixes the unit test failing on macOS.
diff: #include "global_include.h"
  #include "log.h"
  -// To locate QGroundControl plan file during Unit test.
  -#ifdef WINDOWS
  -const std::string SLASH = "\\";
  -#else
  -const std::string SLASH = "/";
  -#endif
  -
  using namespace dronecode_sdk;
  +static const std::string QGC_SAMPLE_PLAN = "plugins/mission/qgroundcontrol_sample.plan";
  +
  struct QGCMissionItem {
  MAV_CMD command;
  std::vector<double> params;
  @@ -79,11 +74,6 @@ TEST(QGCMissionImport, ValidateQGCMissonItems)
  }
  mission_items_local.push_back(new_mission_item);
  - // Locate path of QGC Sample plan
  - std::string self_file_path = __FILE__;
  - std::string self_dir_path = self_file_path.substr(0, self_file_path.rfind(SLASH));
  - const std::string QGC_SAMPLE_PLAN = self_dir_path + SLASH + "qgroundcontrol_sample.plan";
  -
  // Import Mission items from QGC plan
  Mission::mission_items_t mission_items_imported;
  Mission::Result import_result =
  