Fix flaky test: WorkerDecommissionSuite.verify a task with all workers decommissioned succeeds

https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/120284/testReport/junit/org.apache.spark.scheduler/WorkerDecommissionSuite/verify_a_task_with_all_workers_decommissioned_succeeds/

Error Message
java.util.concurrent.TimeoutException: Futures timed out after [2 seconds]
Stacktrace
sbt.ForkMain$ForkError: java.util.concurrent.TimeoutException: Futures timed out after [2 seconds]
at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:259)
at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:187)
at org.apache.spark.SimpleFutureAction.result(FutureAction.scala:130)
at org.apache.spark.util.ThreadUtils$.awaitResult(ThreadUtils.scala:295)
at org.apache.spark.scheduler.WorkerDecommissionSuite.$anonfun$new$3(WorkerDecommissionSuite.scala:73)
at org.scalatest.OutcomeOf.outcomeOf(OutcomeOf.scala:85)
at org.scalatest.OutcomeOf.outcomeOf$(OutcomeOf.scala:83)
at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
at org.scalatest.Transformer.apply(Transformer.scala:22)
at org.scalatest.Transformer.apply(Transformer.scala:20)
at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:186)
at org.apache.spark.SparkFunSuite.withFixture(SparkFunSuite.scala:151)
at org.scalatest.FunSuiteLike.invokeWithFixture$1(FunSuiteLike.scala:184)
at org.scalatest.FunSuiteLike.$anonfun$runTest$1(FunSuiteLike.scala:196)
at org.scalatest.SuperEngine.runTestImpl(Engine.scala:286)
at org.scalatest.FunSuiteLike.runTest(FunSuiteLike.scala:196)
at org.scalatest.FunSuiteLike.runTest$(FunSuiteLike.scala:178)
at org.apache.spark.SparkFunSuite.org$scalatest$BeforeAndAfterEach$$super$runTest(SparkFunSuite.scala:58)