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

[leventov]

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.

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