fix SupervisorHierarchySpec and add more debug output, see #3312

- the children list which was used for the havoc-wreaking part was not
  the right one, leading to an exercise in necromancy
- also print outstanding pings with more detail in case of timeout
Merge pull request #1522 from akka/wip-3312-SupHierS-∂π

fix SupervisorHierarchySpec and add more debug output, see #3312