I agree, and in practice it is what I have been doing for some time, and as I update older modules I tend to convert the ones that were originally fetching tests separately to just use github as the source.
For what it's worth, I've taken the "dl-tests.sh" script that Jamie originally used to download the tests and revamped it to automatically grab the version and github repo from the spec file, find the right git commit hash for that version, and grab the test, support, docs, and examples directories from GitHub. I'm happy to share that script with the list -- or just look at any of my recent SRPMs and you'll see it in there.
One advantage I see to continuing to use registry.npmjs.org as the location is that many of the NodeJS packages seem to switch github repos from time to time. Just in the last few days I've come across three or four packages where this has happened.
That being said, many of you are much more familiar with NodeJS (and NodeJS packaging) than I am -- so I'll defer to the group consensus on this.