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It still looks weird to see code calling methods that obviously don't exist, and with no indication *at the call site* where they come from. Making it clearer with naming would help: "TestThingMixin" or similar.
There are classes like this in the unittest test suite, and I was very confused by them initially until I found where and how they were used. It is obviously *not* a pattern that is widely known for test base classes, as we have this problem of it not being done even in the standard library tests.
In contrast I think code similar to the following would be clear and readable without knowing about multiple inheritance and the mixin trick:
 @test_base_class
 class SomeTestBase(TestCase):
 ...