Make AggregatorTestCase#searchAndReduce(...) more inline with what happens in Search API

[martijnvg]

Currently AggregatorTestCase has its own search logic that unit tests all the aggregations. However this isn't in line with logic in the Search API. Especially with after this recent change: #98204.

Ideally search logic uses in unit tests should be similar with how aggregations get executed in production. However this currently not the case and this causes the unit test coverage to miss certain bugs/issues.

We should investigate whether we can bring the AggregatorTestCase#searchAndReduce(...) more in line with what happens in the Search API. Maybe QueryPhase/AggregatioPhase should be changed to allow unit tests or maybe something else.

[elasticsearchmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[iverase]

After #98715 , this can be closed as now AggregatorTestCase is inline with the search API.