=act #3825 Fix bug in BalancingSpec

* The problem was that the pool had 10 routees and
  depend on blocking to verify balancing scheduling
  to unblocked routee, but the thread pool size was 8
  and therefore routee 8 and 9 was never scheduled
  (blocked) as expected.
Merge pull request #1938 from akka/wip-3825-BalancingSpec-patriknw

=act #3825 Fix bug in BalancingSpec