Improve unit testing coverage

[caryan]

Still to complete:

• [ ] accumulate or averaging snapshots
• [ ] accumulator variance
• [ ] correlators snapshots
• [ ] correlators variance

I think most of these could be handled by slipping on the number of segments with respect to the test pattern generator and take multiple round robins: e.g say there are only 63 segments when there are 64 from the test generators.