Make Catch2 setup more robust against podios in environment

[tmadlener]

BEGINRELEASENOTES

• Add a SKIP_CATCH_DISCOVERY cmake option to skip the unittest discovery of Catch2 to avoid running the catch discovery in an unsuitable environment.
• Make environment for unittests more specific to avoid catching too much of the underlying environment.

ENDRELEASENOTES

Currently this can be quite annoying if there is another podio installation in the environment because the test discovery is then run in an unsuitable environment which can lead to errors during dynamic library loading.

[hegner]

Thanks @tmadlener . Looks fine to me. Should I merge after tests finish/succeed?

[tmadlener]

Sounds good to me. The key4hep nightlies are broken at the moment, python is not picking up the key4hep python . I have adapted the python unittest to use python3 which should work.

(but now github seems to have degraded service, so it might take a while until this appears here)