First, it's not actually illegal, at least in my jurisdiction and in a great many, to make a copy of a copyrighted work.
Secondly, the author somehow gave permission to YouTube to make the video publicly available and downloadable, and YouTube is perfectly happy to provide to me the download URL.
It's illegal in the jurisdiction that Github and the RIAA operate in?
Laws don't exist in a vacuum, so it doesn't seem like a stretch for my comment to refer to the laws actually applicable here right?
Your second point is just remixing the same Stallmanist POV I call out above.
YouTube doesn't just provide you with anything. You go to YouTube, and you find a link, and you stick it in your unit tests.
The fact YouTube's server replies is neither here nor there in the eyes of the law (ironically Youtube does actually return 402 "payment required" and 429 "too many requests" of they figure out what you're doing)
You're free to make whatever esoteric argument you want, but after realizing they did something as silly as put copyrighted videos in the unit tests, there's not much of a leg to stand on.
Especially when there are literally dozens of YT downloader type projects of Github still up right now that didn't do that...
Secondly, the author somehow gave permission to YouTube to make the video publicly available and downloadable, and YouTube is perfectly happy to provide to me the download URL.