Extend reproduction for starvation bug of scaling EsExecutors to work if max pool size > 1

The reproduction of the [EsExecutors starvation bug](https://github.com/elastic/elasticsearch/issues/124667) fixed in [PR](https://github.com/elastic/elasticsearch/pull/124732) should be improved to also reliably test the cases where max pool size > 1.
Currently the reproduction only works if pool size doesn't exceed 1.