Add unit and integration tests for finding expiring documents and upcoming changes

- Implemented comprehensive unit tests for the `load_json_file`, `has_expiring_datetime`, `find_expiring_files`, `print_results`, and `save_results_to_file` functions.
- Added integration tests to validate the complete workflow of finding expiring documents.
- Created unit tests for the `identify_upcoming_changes`, `save_upcoming_file`, `get_doc_ids_for_date`, `get_earliest_pending_date`, and `extract_doc_id_from_filename` functions.
- Included edge case tests to ensure robustness against various scenarios, including invalid date formats and handling of special characters.

Author: marcarl