[libcxx] [test] Fix tests of <cuchar> that unexpectedly succeed on windows

The tests expect that the <cuchar> include should fail. When libc++
is built on top of the MSVC runtime, the header does exist provided
by MSVC. Therefore, just mark the test as unsupported on windows,
to avoid tests that unexpectedly succeed.

Differential Revision: https://reviews.llvm.org/D99096