[libc++][test] Add test coverage for codecvt<char(16|32)_t, char8_t, mbstate_t>

This change adds test coverage for the `codecvt<char16_t, char8_t, mbstate_t>` and `codecvt<char32_t, char8_t, mbstate_t>` ctype facets added to the C++20 WD by [P0482R6](https://wg21.link/P0428R6). Note that libc++ does not implement these facets despite implementing the remainder of P0482, presumably for ABI reasons, so these tests are marked `UNSUPPORTED: libc++`.