libstdc++: Fix fancy pointer test for std::set

The alloc_ptr.cc test for std::set tries to use C++17 features
unconditionally, and tries to use the C++23 range members which haven't
been implemented for std::set yet.

Some of the range checks are left in place but commented out, so they
can be added after the ranges members are implemented. Others (such as
prepend_range) are not valid for std::set at all.

Also fix uses of internal feature test macros in two other tests, which
should use the standard __cpp_lib_xxx macros.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/set/requirements/explicit_instantiation/alloc_ptr.cc:
	Guard node extraction checks with feature test macro. Remove
	calls to non-existent range members.
	* testsuite/23_containers/forward_list/requirements/explicit_instantiation/alloc_ptr.cc:
	Use standard macro not internal one.
	* testsuite/23_containers/list/requirements/explicit_instantiation/alloc_ptr.cc:
	Likewise.