libstdc++: Fix invalid instantiations in tests

libstdc++: Fix invalid instantiations in tests

These tests instantiate std::multiset and std::set with a type that has
no operator< so they should use a custom comparison function.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/multiset/operators/cmp_c++20.cc: Use
	custom comparison function for multiset.
	* testsuite/23_containers/set/operators/cmp_c++20.cc: Use custom
	comparison function for set.