Streamline and document testing mechanism

[pseewald]

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

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

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