[libc++][test] Adjust move_iterator tests to allow C++20

These tests fail due to a couple of changes to `move_iterator` for C++20:

* `move_iterator<I>::operator++(int)` returns `void` in C++20 if `I` doesn't model `forward_iterator`.

* `move_iterator<I>::reference` is calculated in C++20, so `I` must actually have an `operator*() const`.

Differential Revision: https://reviews.llvm.org/D79343