Add regression tests for number of matrix products

We are unlikely going to modify the algorithm too much. Still, if we do, it's important that we don't make it slower than before. For that we can use the counter of multiplications from #93 (closed) (implemented in !62 (merged)) together with pytest-regressions to ensure that its efficiency doesn't drop.

For that we need to keep track of different use cases: different blocks being zero or not, different numbers of perturbations, different user requests.