…duce (elastic#60683)

* Replace AggregatorTestCase#search with AggregatorTestCase#searchAndReduce

This commit removes the ability to test the top level result of an aggregator
before it runs the final reduce. All aggregator tests that use AggregatorTestCase#search
are rewritten with AggregatorTestCase#searchAndReduce in order to ensure that we test
the final output (the one sent to the end user) rather than an intermediary result
that could be different.
This change also removes spurious commits triggered on top of a random index writer.
These commits slow down the tests and are redundant with the commits that the
random index writer performs.