[libc++] [P0919] Some belated review on D87171.

- Simplify the structure of the new tests.
- Test const containers as well as non-const containers,
    since it's easy to do so.
- Remove redundant enable-iffing of helper structs' member functions.
    (They're not instantiated unless they're called, and who would call them?)
- Fix indentation and use more consistent SFINAE method in <unordered_map>.
- Add _LIBCPP_INLINE_VISIBILITY on some swap functions.

Differential Revision: https://reviews.llvm.org/D109011