[CI] TasksClientDocumentationIT testCancelTasks fails in CI

[jaymode]

The TasksClientDocumentationIT#testCancelTasks test failed in CI. I was unable to reproduce this issue but it seems interesting as the task was already cancelled somehow.

Build link: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+intake/203/consoleFull

Reproduce line:

REPRODUCE WITH: ./gradlew :client:rest-high-level:integTestRunner -Dtests.seed=95E5C18ABF7AD273 -Dtests.class=org.elasticsearch.client.documentation.TasksClientDocumentationIT -Dtests.method="testCancelTasks" -Dtests.security.manager=true -Dtests.locale=en-GB -Dtests.timezone=EST -Dcompiler.java=11 -Druntime.java=8

Failure:

java.lang.AssertionError: 
Expected: <[]>
     but: was <[[pz8Vyo5YTxmBo10WkrHmxg][5170] failed, reason [ElasticsearchException[Elasticsearch exception [type=illegal_state_exception, reason=task with id 5170 is already cancelled]]]]>
	at __randomizedtesting.SeedInfo.seed([95E5C18ABF7AD273:727604953DD61AAA]:0)
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
	at org.junit.Assert.assertThat(Assert.java:956)
	at org.junit.Assert.assertThat(Assert.java:923)
	at org.elasticsearch.client.documentation.TasksClientDocumentationIT.testCancelTasks(TasksClientDocumentationIT.java:189)

[elasticmachine]

Pinging @elastic/es-core-infra

[jaymode]

I muted this test in 634bc9d

[hendrikmuhs]

Another failure on 6.x:

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+intake/182/consoleFull

[hendrikmuhs]

I backported this to 6.x: a2087d3

[dakrone]

I recently re-enabled this on master, I'm going to close this for now but if it occurs again we should re-open this and I'll take a look.