libstdc++: Make __gnu_test::default_init_allocator usable in constexpr

libstdc++: Make __gnu_test::default_init_allocator usable in constexpr

If we make this test allocator usable in constant expressions then we'll
get an error if the 'state' data member isn't initialized. This makes it
a more reliable check that allocators are correctly value-initialized
when they're required to be.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/vector/allocator/default_init.cc:
	Add a check using constant evaluation.
	* testsuite/23_containers/vector/bool/allocator/default_init.cc:
	Likewise.
	* testsuite/util/testsuite_allocator.h (default_init_allocator):
	Make all member functions and equality ops constexpr.