=clu #3871 Harden SurviveNetworkInstabilitySpec

* The problem was that the one-way blackhole caused spreading of
  unreachable observations of other nodes from second node, and
  that caused the leader to be able to remove the downed second
  node
* Changed to two-way blackhole and using barrier instead of scheduled
  event to trigger the exceptions when the blackhole was in place
* We should investigate if unreachable observations from downed node
  can be excluded in the convergence check. Created separate ticket for
  that 3875.