As far as I can make out, skipping this test on iOS is as good as we're going to get.

The macOS and iOS langinfo.h headers both define ALT_DIGITS; but experimentally, any request to retrieve the alt digits returns an empty string. The fa_IR subtest is passing because that locale isn't defined, so it's falling back to the previously set locale (en_US). The jp_JP locale does exist, and some locale values (e.g. ABDAY_1) are defined; but ALT_DIGITS values clearly aren't.

I can't find any documentation about this beyond the man page for nl_langinfo, which doesn't shed any light on ALT_DIGITS specifically.