Having a "TestCase factory" would be pretty easy, and solve the scaling problems.

For example:

def make_testcase_classes():
    for backend in backends:
        yield type(
            '{}Test'.format(backend.name),
            (TheBaseClass, unittest.TestCase),
            {'backend': backend}
        )

You would use this in the load_tests function at the module level to generate all the test cases.