How do I use catkin_download_test_data() correctly to run rostests?

Question

I have a working rostest which I was running against a bagfile of image data. Earlier I had stored it in my git package but since its a big file I am thinking of storing the bag file in cloud and downloading while running the test. I came across catkin_download_test_data() to add in CMakelist to do the same but unable to understand how I should modify my rostest to use that. Currently following is the test -

#include "test/test_toll_detector.h"

const auto packagePath = ros::package::getPath("toll_detection");
const auto bagFilePath = packagePath + "/test/bag/toll_detect.bag";
const auto inputImageTopic = "/camera_1/image_raw";

static cv::Mat readFrameFromBag(const std::string& bagFilePath, const std::string& topic) {
  rosbag::Bag bag;
  bag.open(bagFilePath, rosbag::bagmode::Read);
  std::vector<std::string> topics = {topic};
  rosbag::View view(bag, rosbag::TopicQuery(topics));
  cv::Mat frame;
  for (const rosbag::MessageInstance& m : view) {
    sensor_msgs::Image::ConstPtr imgMsg = m.instantiate<sensor_msgs::Image>();
    frame = ImageConverter::convertToCvMat(imgMsg);
    break;
  }
  bag.close();
  return frame;
}

TEST_F(TollDetectorTest, HappyCase) {
  cv::Mat frame = readFrameFromBag(bagFilePath, inputImageTopic);
  cv::Mat image;
  m_HSVFilter_.apply(frame, image);
  std::vector<cv::Rect> tolls = m_TollDetector_.detect(image);
  ASSERT_FALSE(tolls.empty());
}

In the CMakelists I am adding -

if(CATKIN_ENABLE_TESTING)
  find_package(rostest REQUIRED)
  include_directories(${catkin_INCLUDE_DIRS})

  catkin_download_test_data(
    ${PROJECT_NAME}_toll_detector.bag 
    https://<link-to-bag-file>
    DESTINATION ${CATKIN_DEVEL_PREFIX}/${CATKIN_PACKAGE_SHARE_DESTINATION}/test 
    MD5 bce3b89df952233e90fd5f9353fgh2086)

  add_rostest(test/test_all.test DEPENDENCIES ${PROJECT_NAME}_toll_detector.bag)
  catkin_add_gtest(tests_toll_detector
        test/test_main.cpp
        test/test_toll_detector.cpp
      )
      target_link_libraries(tests_toll_detector 
        ${catkin_LIBRARIES}
      )
 endif()

Instead of using packagePath to the rosbag file, how can I modify the code so that I can use catkin_download_test_data() to download the bag file and use the data?