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.
Talk:Selenium/Getting Started/Run tests targeting MediaWiki-Docker
Added to documentation!
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.
Does this specifically require Chrome?
Chromium is fine; that's what WMF CI uses.
Are there no FOSS options?
It should work fine with Chromium too. That's what our continuous integration uses.