Streamline and document testing mechanism

[pseewald]

The current testing mechanism is hard to maintain and should be simplified:

• separate unit tests from tests acting on external Fortran code
• Create at least 3 test suites for unit tests, quick testing and extended testing
• everything should be native Python (no shell scripts or system calls)
• make it easy to add new external Fortran code via configuration file
• Support non-default fprettify options for external Fortran code (Improve testing #44)
• Document how to debug failing tests
• consider switching to pytest (Update testing framework #136)

EDIT: crossed out pytest as unittest seems to be flexible enough for the moment.