From what I read earlier the problem is bound up in the solution. Youtube has mechanisms to prevent download of copyrighted content which youtube-dl circumvents deliberately, which is why it works on every video. That's what the unit tests were all about and why they had to use examples of strictly protected copyrighted material.

The only options are to move development to a server located somewhere beyond the reach of the DMCA and similarly take it off all repositories or to give up on making it possible to download every (or even most) videos from youtube.