libstdc++: Fix testsuite utility's use of allocators

In C++20 the rebind and const_reference members of std::allocator are
gone, so this testsuite utility stopped working, causing
ext/pb_ds/regression/priority_queue_rand_debug.cc to FAIL.

	* testsuite/util/native_type/native_priority_queue.hpp: Use
	allocator_traits to rebind allocator.