Replace paths in tests by pkg resources

[Haydnspass]

Relative paths to file or similiar could likely fail in packaged decode installations (because then folders don't exist anymore?).
Do it similarly to load_reference, i.e.

try:
        import importlib.resources as pkg_resources
    except ImportError: # Try backported to PY<37 `importlib_resources`.
        import importlib_resources as pkg_resources

    from . import reference_files
    param_ref = pkg_resources.open_text(reference_files, 'reference.yaml')