[libc++] Adds a test for std::fill_n.

The function `std::fill` requires a ForwardIterator, but `std::fill_n`
only requires an OutputIterator. Adds a test to validate `std::fill_n`
works with an OutputIterator.

Noticed this while working on LWG3539
format_to must not copy models of output_iterator<const charT&>

Reviewed By: #libc, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D117395