Adds unit tests for functionality added in D12647: Assets: Expose option to reuse data-block data when appending.
For now only testing BKE_asset_library_find_suitable_root_path_from_filepath(). Not sure how to test BKE_asset_library_find_suitable_root_path_from_main() best. It's just a wrapper around the former but takes Main *. I think we'd have to duplicate the test logic, which is not great.