I thought all they need to do is to issue a counter-notice. Popcorn did this and got their code reinstated on github: https://news.ycombinator.com/item?id=24885079
In the worst case, they need to adjust their tests. This is what the issuer of the complaint brought up. Their tests of the code downloaded a couple of seconds of what the RIAA thought is copyrighted material. If I'm not mistaken, this is what they are basing their case on.
If youtube-dl removes those explicit urls from their test cases, I don't think they have a case. You could argue fair use, but remember it's only a test case in that program. The authors could just as easily reprogram the test cases to accept any url a user wants to test.
Anyways, consult lawyers, perhaps change the test code and issue a counter notice and move on.