FJP starvation spec #26973

This doesn't change the fact that the FJP in JDK 11 is less fair but makes the test pass:

* Disable serialize messages to make each message processing less CPU-intense
* Tune the throughput down to the default (was a very high value, 100), so less messages are worked through in one batch of internal dispatch.

akka-actor-tests/src/test/scala/akka/dispatch/ForkJoinPoolStarvationSpec.scala

@@ -10,11 +10,11 @@ import com.typesafe.config.ConfigFactory
 
 object ForkJoinPoolStarvationSpec {
   val config = ConfigFactory.parseString("""
+      |akka.actor.serialize-messages = off
       |actorhang {
-      |
       |  task-dispatcher {
       |    mailbox-type = "akka.dispatch.SingleConsumerOnlyUnboundedMailbox"
-      |    throughput = 100
+      |    throughput = 5
       |    fork-join-executor {
       |      parallelism-factor = 2
       |      parallelism-max = 2