Reachability should check ExtendAllowPrivateAddresses not TestingTorNetwork

When self-testing reachability, use ExtendAllowPrivateAddresses
to determine if local/private addresses imply reachability.

The previous fix used TestingTorNetwork, which implies
ExtendAllowPrivateAddresses, but this excluded rare configs where
ExtendAllowPrivateAddresses is set but TestingTorNetwork is not.

Fixes bug 15771; bugfix on 0.2.6.1-alpha, bug #13924.
Patch by "teor", issue discovered by CJ Ess.