Test framework for calibpipe

Summary

We should provide test coverage for the code. We set a temporary goal of 80% of the code to be covered with the tests. We should follow ctapipe guidelines for testing until new guidelines are released: https://ctapipe.readthedocs.io/en/latest/development/code-guidelines.html#unit-tests

TODO items:

• write unit and integration tests
• ensure the tests are correctly running in the CI
• prepare test coverage metrics (use pycov at the moment)

All new ideas and/or questions to the tests should happen in this issue