[libcxx] [test] Fix the locale ctype widen tests on Windows

[libcxx] [test] Fix the locale ctype widen tests on Windows

On Windows, like on macOS and FreeBSD, widening char(-5) in the
"C" locale succeeds and produces L'\u00fb'.

Switch widen_many to test \xfb instead of \x85; the mingw
version of btowc widens \x85 in the "C" locale into
\u2026 (which is the corresponding character according to the
Windows-1252 codepage), while Microsoft CRT's btowc widens it
into \u0085 (just like macOS and FreeBSD). Switch this to test \xfb
which is the character tested by the widen_1 test (as `char(-5)`),
which gets handled the same by all Windows implementations of btowc.

Differential Revision: https://reviews.llvm.org/D121003