The convention in the stdlib is to name the mixin classes TestXXXBase.  Granted, a lot of those inherit from TestCase.  I have no objection to calling them Mixin instead, I'm just pointing out that there is an existing convention.

(As an aside, when I first ran into the Base pattern it was in a file where the Base did subclass TestCase, and it took me forever to figure out that the Base test wasn't actually getting run.  So a decorator is definitely superior to listing the test cases that actually run elsewhere in the file!)