The intent or action taken in this case is the creation, maintenance, and publication of the tool itself. At issue isn't the intent of any particular end user, or even the majority of end users, but rather of the authors themselves.
To the best of my knowledge, it simply does not matter how the tool is used in practice. AFAIK, all that matters is the intent of the authors in creating and distributing it.
For example I authored and maintain megatools, a tool to download/upload from mega.nz that many people probably use to download the same kind of content that's being the issue with youtube-dl. I have no way of knowing.
I haven't used megatools myself in about 4 years, aside from quick testing prior to release/update. And I never really used mega.nz that much even before. I just wanted to learn a bit about cryptography, and return back to C programming after a few years of just doing PHP/JS, and writing a first mega.nz third-party client was an interesting opportunity at the time. Yet the tool is somewhat popular, and distributed as one of mega clients in various Linux distros.
It's ridiculous to assume intent from some test cases. The only intent I can extrapolate from the actions of authors is that they like to code, fix bugs in other people's code, and want to ease their maintenance burden by having tests. The rest is just speculation.
The tool just sits there on github. Only the tool's users actually do download stuff or attempt to.
Also you'd be hard pressed to prove such specific intent from some random testsuite url, added by one or two authors, when the tool has very many authors/contributors and the README/manual for users just has non-copyrighted material sample urls, anyway.
RIAA just tries to make it harder to preserve regularly disappearing youtube content, like war crimes evidence, just to make some people richer on some stupid music. I can prove their intent, because that's what this tool is used for, and they're trying to remove the tool from the internet. /s This is the same stupid argument they're using. They will fail, but why not try to make other people's lives harder, right?