I like the idea, and I think this would be a useful addition to unittest.

OTOH while this would be applicable to most of the tests (almost every test has a "for" loop to check valid/invalid values, or a few related "subtests" in the same test method), I'm not sure I would use it too often.
The reason is that often there's a single bug that causes the failure(s), and most likely the error reported by unittest is the same (or anyway it's very similar) for all the subtests, so using subtests would just add more noise (at least for me).
However it might be useful in case of sporadic failures or whenever having the full set of failures would be useful to diagnose the problem.