Increase Test Coverage for Core Mapper Classes

Most of the current tests for the various migration tasks (`src/folio_migration_tools/migration_tasks`) are only unit tests for various static methods. To more thoroughly cover this code, we will need to create fixtures and compile test datasets. We should start with the most-used mappers:

- [ ] `mapping_file_transformation/holdings_mapper.py`
- [ ] `mapping_file_transformation/item_mapper.py`
- [ ] `mapping_file_transformation/user_mapper.py`
- [ ] `marc_rules_transformation/marc_file_processor.py`
- [ ] `marc_rules_transformation/marc_reader_wrapper.py`
- [ ] `marc_rules_transformation/rules_mapper_bibs.py`

Start in src/folio_migration_tools/migration_tasks and review the existing unit tests for the six listed mapper and MARC transformation classes. Create fixtures and compile test datasets for holdings_mapper.py, item_mapper.py, user_mapper.py, marc_file_processor.py, marc_reader_wrapper.py, and rules_mapper_bibs.py. Done means these most-used mappers have coverage beyond static methods.

Written by the indexing model from the issue text.