increase barrier-timeout in ClusterShardingSpec, #21718

* In the logs of the failing test we can see that the first node is removed
  as expected and then come back in the membership, which is possible in
  case of conflicting membership state merge. It is supposed to be
  removed again by the auto-down. That doesn't happen within the barrier-timeout.

akka-cluster-sharding/src/multi-jvm/scala/akka/cluster/sharding/ClusterShardingSpec.scala

@@ -158,6 +158,7 @@ abstract class ClusterShardingSpecConfig(
         max-simultaneous-rebalance = 1
       }
     }
+    akka.testconductor.barrier-timeout = 70s
     """))
   nodeConfig(sixth) {
     ConfigFactory.parseString("""akka.cluster.roles = ["frontend"]""")