test_asyncio's `GenericWatcherTests.test_create_subprocess_fails_with_inactive_watcher` is not run

[graingert]

Bug report
GenericWatcherTests does not inherit from TestCase, and so doesn't get run

[Akuli]

There seems to be quite a few classes like this:

$ git grep 'class .*Tests:' Lib/test/ | wc -l
94

[graingert]

There seems to be quite a few classes like this:

$ git grep 'class .*Tests:' Lib/test/ | wc -l
94

I think some of those are supposed to be Mixins, but perhaps they should be re-named *Mixin: instead of *Tests: