Flaky test: SessionHeartbeatWatchdog

SessionHeartbeatWatchdog
- should delete only expired sessions *** FAILED *** (28 milliseconds)
  org.mockito.exceptions.verification.TooManyActualInvocations: testSession$1.stop();
Wanted 1 time:
-> at org.apache.livy.server.interactive.SessionHeartbeatSpec$$anonfun$2$$anonfun$apply$mcV$sp$5.apply$mcV$sp(SessionHeartbeatSpec.scala:83)
But was 2 times. Undesired invocation:
-> at org.apache.livy.sessions.SessionManager.delete(SessionManager.scala:111)
  at org.apache.livy.server.interactive.SessionHeartbeatSpec$$anonfun$2$$anonfun$apply$mcV$sp$5.apply$mcV$sp(SessionHeartbeatSpec.scala:83)
  at org.apache.livy.server.interactive.SessionHeartbeatSpec$$anonfun$2$$anonfun$apply$mcV$sp$5.apply(SessionHeartbeatSpec.scala:68)
  at org.apache.livy.server.interactive.SessionHeartbeatSpec$$anonfun$2$$anonfun$apply$mcV$sp$5.apply(SessionHeartbeatSpec.scala:68)
  at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22)
  at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)
  at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
  at org.scalatest.Transformer.apply(Transformer.scala:22)
  at org.scalatest.Transformer.apply(Transformer.scala:20)
  at org.scalatest.FunSpecLike$$anon$1.apply(FunSpecLike.scala:422)
  at org.scalatest.Suite$class.withFixture(Suite.scala:1122)
  ...