I would test the initTracker function in this way:

// other headers
#include <opencv2/imgcodecs.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/opencv.hpp>

// other tests...

TEST(WebARKitTest, InitTrackerTest) {
  // Create a WebARKitManager object
  webarkit::WebARKitManager manager;
  // Init the manager with the Akaze tracker
  manager.initialiseBase(webarkit::TRACKER_TYPE::AKAZE_TRACKER);
  // Load the test image
  cv::Mat image = cv::imread("pinball.jpg");
  int width = image.cols;
  int height = image.rows;
  unsigned char* data = image.data;
  // Check if initTracker returns sucessfully
  EXPECT_TRUE(manager.initTracker(data, width, height));
}

but i need to provide the necessary opencv headers and libs with openc-em. I will do this first.

i didn't include the openjpeg static lib in the final opencv-em compilation, i will fix this soon.

PostEdit: no it's included even if WITH_JPEG=OFF

with the latest commit tests are passing but cv::imread function can not read the jpeg image, so i developed a workaround providing a fake image only to test the initTracker function. Obviously it's not correct but it's enough to pass this test.
Maybe there is another solution instead of providing opencv includes and libs links (specifying exactly the links...), that is install opencv in the preferred path:

# in opencv/build
cmake .. -dcmake_install_prefix=/home/user/path/to/deps/
cmake --build . --target install

then, configure your project with the same prefix:

# in your project/build
cmake .. -dcmake_prefix_path=/home/user/path/to/deps/

in your cmake files, simply use find_package(opencv)

Another solution is to develop our Docker image with all inside...