Unit tests

[MartinHerren]

Hi Cees,

here a first PR introducing unit tests. Wanted to do more tests but didn't have time yet. So at least a first version until i have more time.

• Adding the CMocka unit test framework
• Adding first tests on the TLE reader wrapper and doing minor fixes detected while writing tests
• Adding github actions so the tests will run when you push changes

I investigated several unit test frameworks. Choice was limited by

• being pure C framework, no need of a C++ compiler
• being standard libs available in every debian/ubuntu system

Tested CMocka, CUnit and Check, found CMocka to be best (modern design, maintained, pretty output, easy and complete testing). If you'd prefer CUnit or Check i could easily switch to them.

The goal was to work on 'displaying satellite names' in rfplot which finally takes a little more work than expected and adding unit tests to the TLE loading code was a good start not to break anything.

Will work on this as soon as i have time and will also try to improve testing. Some parts will require more refactory to become testable.