Fix test_instantiate_when_file_is_in_simpa_home

[faberno]

• added check if code is executed from simpa package
• fixed check against generic paths if a config is already defined in simpa home

Please check the following before creating the pull request (PR):

• [x] Did you run automatic tests?
• [x] Did you run manual tests?
• [x] Is the code provided in the PR still backwards compatible to previous SIMPA versions?

Provide issue / feature request fixed by this PR

Fixes #280