[libc++] add test for latch ctor

Currently, there are no tests to confirm without a doubt the constructor of latch is really constexpr and explicit.
I think this would be an addition that it would not harm to have. In another revision, I was asked to add tests for an almost identical case which made me consider adding this test for the latch class too.

Reviewed By: #libc, philnik

Spies: philnik, libcxx-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D154957