[libc++] Refactor the tests for [iterator.range] (#67496)

The tests were a bit of a mess -- the testing coverage wasn't bad but it
was extremely difficult to see what was being tested and where. I split
up the tests to make them easier to audit for completeness and did such
an audit, adding a few missing tests (e.g. the conditional noexcept-ness
of std::cbegin and std::cend). I also audited the synopsis and adjusted
it where it needed to be adjusted.

This patch is in preparation of fixing #67471.