[libc++][chrono] Fixes year_month year wrapping. (#74938)

Adding months to a year_month should wrap the year when the month
becomes greater than twelve or less than one.

This fixes the issue for year_month. Other classes with a year and month
do not have this issue. This has been verified and tests are added to
avoid possible regressions.

Also fixes some variable copy-paste errors in the tests.

Fixes https://github.com/llvm/llvm-project/issues/73162