Automatic AggregatorFactory tests

[leventov]

Would be nice to be able to automatically recognize new AggregatorFactory implementations in the project and run some automatic tests, such as that their comparators accept nulls, getCombiningFactory().getCombiningFactory() equals to getCombiningFactory(), etc.

[leventov]

Also that factory.getMergeFactory(factory) is equal to factory.getCombiningFactory().

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been closed due to lack of activity. If you think that is incorrect, or the issue requires additional review, you can revive the issue at any time.