[libc++] Fix rvalue_iterator arithmetic operators in tests (#220686)

`rvalue_iterator` holds an `it_` member, not an `it`. This issue was
found while force-instantiating the template members. In current tests
the problematic code seems to be unreachable.

GitOrigin-RevId: 4c4b4b93c9ac359acc3782a92fc7efe204f218c5