Treat timeout of all name lookup tests as also meaning that DNS is broken
Right now, AFAICT, our DNS testing code only looks for hijacking: the case where all names (or just NXDOMAIN names) get redirected to a small set of IPs. But it doesn't actually check for cases where all the names we lookup get timeouts or fail. We should probably detect that too, and also treat that as meaning that our DNS is broken.