abort() currently only changes that remote-deployed child actors are not
waited for during termination (because that would not change anything);
it is still a different operation than shutdown() since it changes what
you are guaranteed to observe after termination.
testConductor.shutdown(..., abort = true) uses this mode of termination.
improve MultiNodeSpec to allow injection of deployment configuration
into arbitrary actor systems
include number of received elements in the timeout failure message for
TestKit.receiveN
