[libc++][test] Consistently use `TEST_SHORT_WCHAR` (#74958)

Found while running libc++'s test suite with MSVC's STL.

* In `escaped_output.unicode.pass.cpp`, replace `_LIBCPP_SHORT_WCHAR`
with `TEST_SHORT_WCHAR`.
+ This was the only test that was directly using the `_LIBCPP` macro.
`libcxx/test/support/test_macros.h` performs this mapping:
https://github.com/llvm/llvm-project/blob/c60ac509399da5cba533b9b6cc5b983c59f7d7b3/libcxx/test/support/test_macros.h#L442-L444
* In `msvc_stdlib_force_include.h`, define `TEST_SHORT_WCHAR`.