Dir auths should stop testing reachability of their DirPort

This is the tiny piece of #40282 (closed) that needs to be done soon: we've put patches in place so that relays won't allow exiting to the listed dir auth dirports, yet the dir auths use exactly this technique to test their reachability, and they currently don't publish their descriptor if they can't find their dirport reachable.

This surprise happened in practice during the "dirport ddos via exits" event, where moria1 kept complaining that its dirport wasn't reachable, and I assume that meant it wasn't ever planning to publish a new descriptor, which meant it would eventually fall out of the consensus (which isn't the end of the world either, since it doesn't need to be in the consensus for most of its functions).

If we were doing #40282 (closed) in 0.4.5 and doing the "leave the dirport listed but just stop testing it" version of that ticket, we could just do that and it would accomplish this ticket too. But my guess is that we want to do that ticket for 0.4.6, in which case we need to do some minimal thing in 0.4.5 before we get there?