Don't do reachability testing over IPv6 when AuthDirHasIPv6Connectivity isn't set
Authorities (directory and bridge) set up outgoing IPv6 connections
even when AuthDirHasIPv6Connectivity is 0.
This is because we end up in dirserv_single_reachability_test() from
routerlist_descriptors_added() when loading descriptors from disk at
startup.
issue