I guess I'm not really done talking about this, though my bow to you as maintainer still stands.

The mixin tests *can't* be run in isolation, that's the whole point.  Otherwise you could just let unittest run them, and wouldn't need to mark them as not-really-a-TestCase.  The typical mixin test class uses attributes that don't exist except on the concrete TestCases, so I'm not sure why calling methods that don't exist on the mixin is any worse :)