nction given by ``test_fn`` as a subtest according to the
    configurations specified by ``subtest_config``. This amortizes the
    costly setup overhead (including process spawn and initializing the
    process group) over the subtests.

    Args:
        subtest_config (Dict[str, List[Any]]): A mapping from subtest
            keyword argument name to a list of its possible values.
        test_fn (Callable): A callable that runs the actual test.
        test_args: Positional arguments to pass to ``test_fn``.
        test_kwargs: Keyword arguments to pass to ``test_fn``.
    c