Remove special authority semantics for AssumeReachable.

AssumeReachable should only be about whether a relay thinks that it is reachable itself. But previously, we've had it also turn off reachability checking of other relays on authorities. (Technically, reachability tests were still run, but we would ignore the results.)

With this patch, there is a new AuthDirTestReachability option (default 1) that controls whether authorities run reachability tests.

Making this change allows us to have test networks where authorities set AssumeReachable without disabling their reachability testing entirely.

Closes ticket #34445 (closed).