I also had to have ffmpeg installed to get the tests to run. I'd add it to this page, but maybe it's just a quirk of my system not having already installed it along with something else.
Topic on Talk:Selenium/Getting Started/Run tests targeting MediaWiki-Docker
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Thanks for letting me know and apologies for the late reply. ffmpeg was added recently because of video recording (T179188) and I have forgot to update the documentation.