[Test] QueryStringQueryBuilderTests fails occasionally

[cbuescher]

Just found this while running tests locally. This tests fails reproducibly with the given seed:

./gradlew :server:test \
  -Dtests.seed=3BB9F91BD6345C6C \
  -Dtests.class=org.elasticsearch.index.query.QueryStringQueryBuilderTests

I think I already see the problem and will open a PR with a fix soon.

[elasticmachine]

Pinging @elastic/es-search-aggs