CI: IndexShardTests#testDocStats fails

[dnhatn]

This test started failing after we upgraded to Lucene-7.5.

I can reproduce this locally.

./gradlew :server:test \
  -Dtests.seed=B201D9968D50FB58 \
  -Dtests.class=org.elasticsearch.index.shard.IndexShardTests \
  -Dtests.method="testDocStats" \
  -Dtests.security.manager=true \
  -Dtests.locale=sr-CS \
  -Dtests.timezone=Asia/Makassar

CI: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob-windows-compatibility/1785/console

[elasticmachine]

Pinging @elastic/es-distributed

[dnhatn]

I've muted this test.