[libc++][test] Adds transcode option. (#73395)

This should make it easier to get better output when wchar_t tests fail.
The code is based on the Unicode transcoding in `<format>`.

Differential Revision: https://reviews.llvm.org/D150593

GitOrigin-RevId: 965fe352a770134968538cf9a0818016b7c3c7f6