[CI] WebhookIntegrationTests teardown failure

[martijnvg]

Reproduce line:

REPRODUCE WITH: ./gradlew ':x-pack:plugin:watcher:test' --tests "org.elasticsearch.xpack.watcher.actions.webhook.WebhookIntegrationTests"
-Dtests.seed=540CAA169039113A
-Dtests.security.manager=true
-Dtests.locale=en-US
-Dtests.timezone=Etc/UTC
-Dcompiler.java=12
-Druntime.java=13

Does not reproduce for me locally.

Test suite failed due to uncaught exception:

> WARNING: Uncaught exception in thread: Thread[elasticsearch[node_s1][snapshot][T#2],5,TGRP-WebhookIntegrationTests] |  
-- | --
  | 2> java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask@345c0ad7[Not completed, task = java.util.concurrent.Executors$RunnableAdapter@4f070870[Wrapped task = org.elasticsearch.xpack.core.scheduler.SchedulerEngine$ActiveSchedule@a059ff4]] rejected from java.util.concurrent.ScheduledThreadPoolExecutor@3bc5777e[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0] |  
  | 2> 	at __randomizedtesting.SeedInfo.seed([540CAA169039113A]:0) |  
  | 2> 	at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2057) |  
  | 2> 	at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:827) |  
  | 2> 	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor.delayedExecute(ScheduledThreadPoolExecutor.java:340) |  
  | 2> 	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor.schedule(ScheduledThreadPoolExecutor.java:562) |  
  | 2> 	at org.elasticsearch.xpack.core.scheduler.SchedulerEngine$ActiveSchedule.scheduleNextRun(SchedulerEngine.java:222) |  
  | 2> 	at org.elasticsearch.xpack.core.scheduler.SchedulerEngine$ActiveSchedule.<init>(SchedulerEngine.java:196) |  
  | 1> [2019-12-18T18:02:46,555][INFO ][o.e.n.Node               ] [suite] closed |  
  | 2> 	at org.elasticsearch.xpack.core.scheduler.SchedulerEngine.add(SchedulerEngine.java:147) |  
  | 2> 	at org.elasticsearch.xpack.slm.SnapshotRetentionService.rescheduleRetentionJob(SnapshotRetentionService.java:90) |  
  | 2> 	at org.elasticsearch.xpack.slm.SnapshotRetentionService.onMaster(SnapshotRetentionService.java:75) |  
  | 2> 	at org.elasticsearch.cluster.service.ClusterApplierService$OnMasterRunnable.run(ClusterApplierService.java:650) |  
  | 2> 	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:703) |  
  | 2> 	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130) |  
  | 2> 	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630) |  
  | 2> 	at java.base/java.lang.Thread.run(Thread.java:832)

Build failure: https://gradle-enterprise.elastic.co/s/hjvfr234rxj5m/

I think this caused by a timing issue and all tests extending from AbstractWatcherIntegrationTestCase are prone this this. Maybe we should disable slm in this test base class?

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Watcher)