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
Revert "[TEST] Disable dummy documents for QueryRescorerTests#testEquivalence"

This reverts commit 1bee019371b1776a8c443f1474a4a9358c96df45.
Revert "[TEST] Provide overloaded indexRandom to opt out of dummy documents"

This reverts commit ee43803486df879354fa0a79cd67455ebcc8dec2.
Revert "Add dummy docs injection to indexRandom"

This reverts commit beffc1893500cca102e9ed58d4e57030a32000f0.
better reproduce https://github.com/elasticsearch/dev/issues/207