Fix race in CircuitBreakerMTSpec.openBreaker, see #2953

* The problem was that the breaker is asynchronous so it may
  take some extra time to open.
* Was able to reproduce with a sleep in onComplete in
  CircuitBreaker L303
* Added an extra awaitCond in case the normal (quick)
  failing calls don't open the breaker
Merge pull request #1082 from akka/wip-2953-openBreaker-patriknw

Fix race in CircuitBreakerMTSpec.openBreaker, see #2953