libstdc++: Fix __gnu_test::input_iterator_wrapper::operator++(int)

I noticed recently that our input_iterator_wrapper utility for writing
tests has the following post-increment operator:

    void
    operator++(int)
    {
      ++*this;
    }

That fails to meet the Cpp17InputIterator requirement that *r++ is
valid. This change makes it return a non-void proxy type that can be
deferenced to produce another proxy, which is convertible to the
value_type. The second proxy converts to const T& to ensure it can't be
written to.

	* testsuite/util/testsuite_iterators.h:
	(input_iterator_wrapper::operator++(int)): Return proxy object.