[libc++] Refactor the tests for std::prev, next and advance

This started as an attempt to fix a GCC 11 warning of misplaced parentheses.
I then noticed that trying to fix the parentheses warning actually triggered
errors in the tests, showing that we were incorrectly assuming that the
implementation of ranges::advance was using operator+= or operator-=.

This commit fixes that issue and makes the tests easier to follow by
localizing the assertions it makes.

Differential Revision: https://reviews.llvm.org/D103272