[libc++] [LWG3221] Add tests for wrapping operator+(year_month, months).

The behaviour didn't change since commit 5b08c1742a536f54bd5e270b00ff851cbc7314ef (Recommit <chrono> changes with a couple xtra tests marked to fail on apple's clang.)

* http://wg21.link/lwg3221

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D92730

GitOrigin-RevId: 877170f3eb2a7f288eca1e226e5a07d607f96474
2 files changed