When DisableNetwork is 1 but !circuit_enough_testing_circs(), we can still launch circuits

In consider_testing_reachability(), we check

  if (test_or && (!orport_reachable || !circuit_enough_testing_circs())) {

Once legacy/trac#18616 (moved) is merged, the first function will return 1 for orport_reachable when DisableNetwork is 1, so that bug will go away.

But it will remain the case that if !circuit_enough_testing_circs(), we will proceed to call circuit_launch_by_extend_info(), even when DisableNetwork is 1.

There are four places that call consider_testing_reachability():

• circuitbuild.c:circuit_send_next_onion_skin()
• circuituse.c:circuit_testing_opened()
• main.c:directory_info_has_arrived()
• main.c:check_for_reachability_bw_callback()

I think the middle two are safe, since they shouldn't happen while DisableNetwork is set.

I think the first one is iffy, since it's called from a bunch of places so I'm not sure, but given the name I hope it doesn't get called during DisableNetwork.

And I think the fourth one is bad news, since it gets called periodically and doesn't check DisableNetwork.