fix NodeChurnSpec tombstones, #23369

* the gossip was growing because we introduced tombstones
* in this test it should be safe to have a short removal period
  of the tombstones

akka-cluster/src/multi-jvm/scala/akka/cluster/NodeChurnSpec.scala

@@ -23,6 +23,7 @@ object NodeChurnMultiJvmSpec extends MultiNodeConfig {
   commonConfig(debugConfig(on = false).
     withFallback(ConfigFactory.parseString("""
       akka.cluster.auto-down-unreachable-after = 1s
+      akka.cluster.prune-gossip-tombstones-after = 1s
       akka.remote.log-frame-size-exceeding = 1200b
       akka.remote.artery.advanced {
         idle-cpu-level = 1