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

The behaviour didn't change since commit 5b08c174 (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