[libc++] Use cpp20_output_iterator in tests.

Adds the new cpp20_output_iterator in the ranges::transform test.

Reviewed By: philnik, #libc

Differential Revision: https://reviews.llvm.org/D123139

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

@@ -571,6 +571,7 @@ constexpr bool test() {
 
 int main(int, char**) {
   test_iterators_in1_in2<cpp17_output_iterator<int*>>();
+  test_iterators_in1_in2<cpp20_output_iterator<int*>>();
   test_iterators_in1_in2<forward_iterator<int*>>();
   test_iterators_in1_in2<bidirectional_iterator<int*>>();
   test_iterators_in1_in2<random_access_iterator<int*>>();