Verify that our checks for poisson function calls with scalar lambda are correct

[denisalevi]

@mstimberg When you are back, could we have a quick look at my check for whether poisson(lambda) uses a lambda that is constant for a run or not? Would be great :)

For details, see my comment in my poisson function PR.

All tests are passing and I think poisson implementation is done. Just leaving this issue here open to double check I didn't miss anything that isn't tested.