[libc] Clean up errno header usage in wchar tests. (#157942)

See PR #157898 and PR #157517 for similar changes to math tests.

Don't include libc_errno.h header, since the tests don't manipulate
errno directly. Instead, where appropriate, include hdr/errno_macros.h
proxy header to get errno constant values to match against.

GitOrigin-RevId: d7318ebe9338cd2d4cd4023bd308d981b5e01ece
11 files changed