re-add sha script; it's used in the Makefile While there, use it in the tests too

rework the regression suite The tests are still there, the suite is equivalent to the old one, but this one is better structured. The biggest annoyance I had with the old one was that it wasn't straightforward to test only a specific set of tests. It's still impossible, but it's way easier to do it now. This extract all the tests to their own functions. It's overall better in all possible regards.