Added some tests, minor refactoring

Hey Jeff,

here is my initial set of changes. I figured I'd file a pull request early, instead of veering off completely -- so you can give me feedback whether the style/quality of the changes matches your requirements. Quick summary:

- added tests for MPIX_Sendrecv_x() and MPIX_Bcast_x()
- tests now accept two command line parameters: the new, 2nd parameter allows the user to specify a remainder
- added test target for convenience
- moved common verification code to dedicated file