[libcxx] reworks invocable and regular_invocable tests

The tests for `std::invocable` and `std::regular_invocable` were
woefully incomplete. This patch closes many of the gaps (though some
probably remain).

Differential Revision: https://reviews.llvm.org/D99398

GitOrigin-RevId: e06f1a8e3cc60bb8d2a937f47f6dac06a8818a17
3 files changed