EOU: Implement regression self-testing for all xrpn commands

[acser123]

I know this is not easy or trivial, but I put it out there :) It would be nice to have a command in or even better outside of xrpn that ran a self test of all commands to ensure they work fine and there are no regressions (new fixes breaking stuff that has been working fine). This is because xrpn is a very complex system and commands are the basic building blocks of HP 41 programs. This self test should be extensible with new features.