libstdc++: Add a test that takes the split_view of a non-forward range

libstdc++: Add a test that takes the split_view of a non-forward range

This adds a tests that verifies taking the split_view of a non-forward range
works correctly.  Doing so revealed a typo in one of _OuterIter's constructors.

It also revealed that the default constructor of
__gnu_test::test_range::iterator misbehaves, because by delegating to
Iter<T>(nullptr, nullptr) we perform a null-pointer deref at runtime in
input_iterator_wrapper's constructor due to the ITERATOR_VERIFY check therein.
Instead of delegating to this constructor it seems we can just inherit the
protected default constructor, which does not contain this ITERATOR_VERIFY
check.

libstdc++-v3/ChangeLog:

	* include/std/ranges (split_view::_OuterIter::_OuterIter): Typo fix,
	'address' -> 'std::__addressof'.
	* testsuite/std/ranges/adaptors/split.cc: Test taking the split_view of
	a non-forward input_range.
	* testsuite/util/testsuite_iterators.h (output_iterator_wrapper): Make
	default constructor protected instead of deleted, like with
	input_iterator_wrapper.
	(test_range::iterator): Add comment explaining that this type is used
	only when the underlying wrapper is input_iterator_wrapper or
	output_iterator_wrapper.  Remove delegating defaulted constructor so
	that the inherited default constructor is used instead.