Allow customizer for test

[furstenheim]

Basically this pr allows to customize the aggregation. For example tracking the points that are aggregated (there was a todo for this, I've added a test).

I don't know much golang, so it is possible that some are the default way to do it. Feel free to point them out and I'll try to fix them. For example, there were a couple of test utils that I duplicated, I suppose there should be a better way.