Add a basic Catch2 based unittest setup

[tmadlener]

BEGINRELEASENOTES

• Adding a basic Catch2 v3 based unittest setup that can be used to easily add unittests.
  • The CMake configuration allows to either build Catch2 on the fly or to discover a suitable installation of Catch2
• Add the existing test and an example unittest to the test suit that can be run via ctest after building the package.
• Disable building the tests for the coverity workflow for now to avoid polluting the output of that with Catch2 issues

ENDRELEASENOTES

This serves as the basis for the discussion / plan described in #24 (comment)

[tmadlener]

@andresailer I am not entirely sure why, but it seems git is not working when using the iLCSoft nightlies in CI: https://github.com/tmadlener/MarlinUtil/actions/runs/3113369470/jobs/5047912751#step:4:328

Is this something that can be easily fixed?