[libcxx] [test] Fix the monetary locale pos/neg_format test for Windows and macOS

The zh_CN.UTF-8 locale on Glibc has got `n_sign_posn == 4` (which means
having the negative sign just after the currency symbol), but has
`int_n_sign_posn == 1` (which means before the string).

On Windows, there's no separate `int_n_sign_posn` field, so the same
`n_sign_posn` (which is 4 there too) is used for international currency
formatting too. This makes the ordering for the international case on
Windows be the same as for the national one right above it.

On Apple platforms, the fr_FR.UTF-8 locale has got `n_sign_posn == 2`
but `p_sign_posn == 1`, giving a different order for the French locale
for the negative format.

On Apple platforms for the zh_CN.UTF-8 locale, both `n_sign_posn` and
`int_n_sign_posn` are 4, but `p_sign_posn` and `int_p_sign_posn` are 1.

Differential Revision: https://reviews.llvm.org/D120550