After reading the link you provided I noticed that we can also provide an mjpeg.
Depending on what your test requirements – this may be sufficient for you. As a terminal command with ffmpeg installed:
ffmpeg -i oldfile.mp4 newfile.mjpeg
then I tested by running Google Chrome from the terminal using:
google-chrome --use-fake-device-for-media-stream --use-file-for-fake-video-capture=newfile.mjpeg
After navigating to Tracking JS I could see the video being played back.
I hope that works for you!