I think when I started this patch I was not aware of the existing test. Then I looked hard at the structure of all those tests and it seemed to me being obvious that a test for `utils/formats.py` should live in `utils_tests/test_formats.py`.

I'm a bit confused now about what to do next. I could either turn the tests here into methods on the existing `i18n.tests.FormattingTests` test case or I could move the tests `test_iter_format_modules` and `test_iter_format_modules_stability` into my new class here.

What do you suggest?