[libc++] Use cpp20_output_iterator in tests.

[Mordante]

Summary

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

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

constexpr bool test() {
     }
   }
 
   return true;
 }
 
 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*>>();
   test_iterators_in1_in2<contiguous_iterator<int*>>();
   test_iterators_in1_in2<int*>();
   test();
   static_assert(test());
 
   return 0;
 }