Facilitate testing AggregatorFactory (e. g. its makeAggregateCombiner() method) and AggregateCombiner

`AggregateCombiner` is currently hard to test because it accepts works with `ColumnValueSelector` containers rather than with simple object values. At very least, a "singleton" implementation of `ColumnValueSelector` should be provided. Better, there should be something like a mini-framework for testing different cases about aggregator factories. This framework should pay off well because there are so many AggregatorFactories in the codebase.

Related to #7329.

Start by reading AggregatorFactory, its makeAggregateCombiner() method, AggregateCombiner, and ColumnValueSelector, then review related issue #7329. Done means a singleton ColumnValueSelector is available at minimum, with a reusable testing approach for different aggregator-factory cases if the broader framework is pursued.

Written by the indexing model from the issue text.