check-qjson: Add test for JSON nesting depth limit

This would have prevented the regression mentioned in the previous
commit.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1448486613-17634-4-git-send-email-armbru@redhat.com>
