[libc++][NFC] Use cpp17_output_iterator in tests.

The renames the output_iterator to cpp17_output_iterator. These
iterators are still used in C++20 so it's not possible to change the
current type to the new C++20 requirements. This is done in a similar
fashion as the cpp17_input_iterator.

Reviewed By: #libc, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D117950