Add dummy docs injection to indexRandom

This commit add `dummy docs` to `ElasticsearchIntegrationTest#indexRandom`.
It indexes document with an empty body into the indices specified by the docs
and deletes them after all docs have been indexed. This produces gaps in
the segments and enforces usage of accept docs on lower levels to ensure
the features work with delete documents as well.
[TEST] Provide overloaded indexRandom to opt out of dummy documents
[TEST] Disable dummy documents for QueryRescorerTests#testEquivalence