test_ukify: use Path-based fixtures

test_ukify: use Path-based fixtures

Quoting https://docs.pytest.org/en/stable/how-to/tmp_path.html#the-default-base-temporary-directory:
> The tmpdir and tmpdir_factory fixtures are similar to tmp_path and
> tmp_path_factory, but use/return legacy py.path.local objects rather than
> standard pathlib.Path objects.
>
> These days, it is preferred to use tmp_path and tmp_path_factory.