tests: Cover input visit beyond end of list

When you try to visit beyond the end of a list, the qobject input
visitor crashes, and the string visitor screws returns garbage.  The
generated list visits never go beyond the list end, but manual visits
could.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1488544368-30622-27-git-send-email-armbru@redhat.com>
