=con Harden ClusterShardingSpec some more

* Replace sleep with awaitAssert
* Use separate probes for awaitAssert checks to avoid spill-over
  to the testActor
* Some additional cleanup
* Deliver buffered messages when HostShard is received
  Test failures showed that initial messages could be re-ordered otherwise
Merge pull request #15734 from akka/wip-harden-ClusterShardingSpec-patriknw

=con Harden ClusterShardingSpec some more