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.
