[CI] ClusterDisruptionIT#testAckedIndexing failure on 6.6

[cbuescher]

Build: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.6+periodic/281/console

Reproduce

./gradlew :server:integTest \
  -Dtests.seed=28EA74D48DC5F83F \
  -Dtests.class=org.elasticsearch.discovery.ClusterDisruptionIT \
  -Dtests.method="testAckedIndexing" \
  -Dtests.security.manager=true \
  -Dtests.locale=zh-SG \
  -Dtests.timezone=Asia/Almaty \
  -Dcompiler.java=11 \
  -Druntime.java=8

I kept this running in a loop for about 15 repetiotions locally before I got the first failure. Looks like this then:

   > Throwable #1: java.lang.AssertionError: [test][1], node[L-28xZP7Q0WyfVS1JrcTHA], [R], s[STARTED], a[id=EX57TRsNTX-vc3tfmteydQ] seq_no_stats mismatch
   > Expected: <SeqNoStats{maxSeqNo=4, localCheckpoint=4, globalCheckpoint=4}>
   >      but: was <SeqNoStats{maxSeqNo=4, localCheckpoint=4, globalCheckpoint=1}>
   >    at __randomizedtesting.SeedInfo.seed([28EA74D48DC5F83F:A22BC027D1B81E74]:0)
   >    at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
   >    at org.elasticsearch.test.InternalTestCluster.lambda$assertSeqNos$8(InternalTestCluster.java:1328)
   >    at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:848)
   >    at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:822)
   >    at org.elasticsearch.test.InternalTestCluster.assertSeqNos(InternalTestCluster.java:1298)
   >    at org.elasticsearch.discovery.AbstractDisruptionTestCase.beforeIndexDeletion(AbstractDisruptionTestCase.java:104)
   >    at org.elasticsearch.test.ESIntegTestCase.afterInternal(ESIntegTestCase.java:585)
   >    at org.elasticsearch.test.ESIntegTestCase.cleanUpCluster(ESIntegTestCase.java:2199)
   >    at java.lang.Thread.run(Thread.java:748)
   >    Suppressed: java.lang.AssertionError: [test][1], node[L-28xZP7Q0WyfVS1JrcTHA], [R], s[STARTED], a[id=EX57TRsNTX-vc3tfmteydQ] seq_no_stats mismatch
   > Expected: <SeqNoStats{maxSeqNo=4, localCheckpoint=4, globalCheckpoint=4}>
   >      but: was <SeqNoStats{maxSeqNo=4, localCheckpoint=4, globalCheckpoint=1}>
   >            at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
   >            at org.elasticsearch.test.InternalTestCluster.lambda$assertSeqNos$8(InternalTestCluster.java:1328)
   >            at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:836)
   >            ... 40 more

This is also what I see in the build logs.

[cbuescher]

@ywelsch I'll keep this unmuted for now since I'm not sure this is an issue related to a particular test, let me know if you think this is specific enough so that muting would work.