Test <functional> objects which had their result_types removed in C++20

test/Jamfile.v2

@@ -74,3 +74,4 @@ run bind_noexcept_test.cpp ;
 run bind_noexcept_mf_test.cpp ;
 run global_placeholders.cpp ;
 run mem_fn_noexcept_test.cpp ;
+run bind_cpp20_test.cpp ;