fix: outdated test test_compare_against_str

[hf-kklein]

I don't know if this is the actually expected behaviour but at least the test passes now. I'd strongly recommend running all tests in the CI/a PR always and not commiting or pushing directly to master to prevent broken tests on the default/main/master branch or accidentially releasing with broken/outdated tests.

there are other broken tests:

                      raise ValidationError(

                           f"{self.tag} Segment, pos. {index}: {e}"
                       )

E pydifact.exceptions.ValidationError: UNB Segment, pos. 3: Tag 0017 (Date) must be numeric and must have 6 digits. Current value: '07042029'

where the test setup is already errornous - no idea what is the new expected behaviour there.

If the failing Tests are intentional, you might use @pytest.skip("an explanation goes here") decorator for the test function