[libc++] Refactor the tests for ranges::{advance,next,prev}

This makes all the tests consistent and improves code coverage. This also
uncovers a bug with negative indices in advance() (which also impacts
prev()) -- I'll fix that in a subsequent patch.

I chose to only count operations in the tests for ranges::advance because
doing so in prev() and next() too was reaching diminishing returns, and
didn't meaningfully improve our test coverage.