TermsGroupSourceTests.testSupportsIncrementalBucketUpdate is flaky

[rjernst]

Build scan:
https://gradle-enterprise.elastic.co/s/et6j32wppwsqk

Repro line:
./gradlew ':x-pack:plugin:core:test' --tests "org.elasticsearch.xpack.core.transform.transforms.pivot.TermsGroupSourceTests.testSupportsIncrementalBucketUpdate" -Dtests.seed=8ED60956387B79DC -Dtests.security.manager=true -Dtests.locale=ca-ES -Dtests.timezone=America/Monterrey -Druntime.java=11

Reproduces locally?: yes

Applicable branches: master, 7.x

Failure excerpt:

  2> java.lang.AssertionError: expected:<false> but was:<true>
        at __randomizedtesting.SeedInfo.seed([8ED60956387B79DC:EA838C26671EEB47]:0)
        at org.junit.Assert.fail(Assert.java:88)
        at org.junit.Assert.failNotEquals(Assert.java:834)
        at org.junit.Assert.assertEquals(Assert.java:118)
        at org.junit.Assert.assertEquals(Assert.java:144)
        at org.elasticsearch.xpack.core.transform.transforms.pivot.TermsGroupSourceTests.testSupportsIncrementalBucketUpdate(TermsGroupSourceTests.java:53)

I encountered this test failing in a PR, and found it reproduced on master, inconsistently. I ran with 20 iterations and got 8 failures.

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)