Add regression tests for (intrinsic) calibration

Our current regression tests do not check calibration in any way. We should remedy that.

Since using the GUI in a regression test via a tool like PyAutoGUI would likely be brittle, I'll propose using the CLI to calibrate with test files (using demo). This will check calibration, saving in XML and loading from XML.

The goal of the tests is more to see that we do sth., less to check the results. That probably is tested by OpenCV.