[libc++][test] Disable test for extension that's unsupportable in C++20

Defining the nested types `reference` and `iterator_concept` of `reverse_iterator<I>` necessarily requires `I` to be complete in C++20. These tests that verify that `std::map<int, X>::reverse_iterator` can be instantiated when `X` is incomplete are going to have a bad time.

Differential Revision: https://reviews.llvm.org/D78944