SignificantTextAggregatorTests#testSignificance fails in CI

[danielmitterdorfer]

• Example failure: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob-unix-compatibility/os=fedora/1174/console
• Failure:

FAILURE 0.08s J0 | SignificantTermsAggregatorTests.testSignificance <<< FAILURES!
   > Throwable #1: java.lang.AssertionError: expected:<1> but was:<2>
   > 	at __randomizedtesting.SeedInfo.seed([1FDF8F41EC94A0D5:F341B471691055D5]:0)
   > 	at org.elasticsearch.search.aggregations.bucket.significant.SignificantTermsAggregatorTests.testSignificance(SignificantTermsAggregatorTests.java:124)
   > 	at java.lang.Thread.run(Thread.java:748)

• Reproduction line: gradle :core:test -Dtests.seed=1FDF8F41EC94A0D5 -Dtests.class=org.elasticsearch.search.aggregations.bucket.significant.SignificantTermsAggregatorTests -Dtests.method="testSignificance" -Dtests.security.manager=true -Dtests.locale=id -Dtests.timezone=America/North_Dakota/Beulah
• Frequency: Reproduces always with the above reproduction line