[libc++][ranges][NFC] Test new requirements for `basic_string_view` and `span` iterators.

Note that most changes to `strings` and `views.span` from the One Ranges
Proposal are no longer applicable:
- free `begin` and `end` functions taking `basic_string_view` and `span`
  were removed by [P1870](http://wg21.link/p1870);
- `span::const_iterator` was removed by [LWG3320](https://cplusplus.github.io/LWG/lwg-defects.html#3320).

Reviewed By: #libc, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D118687