[CI] LuceneChangesSnapshotTests#testUpdateAndReadChangesConcurrently fails

[jimczi]

LuceneChangesSnapshotTests#testUpdateAndReadChangesConcurrently fails occasionally on master and 6x. I reproduce locally with the following seed (the failure doesn't occur on each run though but I was able to reproduce after a few ones):

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+release-tests/1005/console

./gradlew :server:test \
  -Dtests.seed=95ED5AB5FEF63FF6 \
  -Dtests.class=org.elasticsearch.index.engine.LuceneChangesSnapshotTests \
  -Dtests.method="testUpdateAndReadChangesConcurrently" \
  -Dtests.security.manager=true \
  -Dbuild.snapshot=false \
  -Dtests.jvm.argline="-Dbuild.snapshot=false" \
  -Dtests.locale=bg \
  -Dtests.timezone=Pacific/Enderbury \
  -Dcompiler.java=10 \
  -Druntime.java=8

[elasticmachine]

Pinging @elastic/es-core-infra

[jimczi]

I muted the test in master and 6.x:
2fa75b4

[elasticmachine]

Pinging @elastic/es-distributed