Increase Test Coverage for Core Migration Tasks

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, compile test datasets, and mock mapper classes. We should start with the most-used tasks:

- [x] `bibs_transformer.py`
- [x] `holdings_marc_transformer.py`
- [ ] `holdings_csv_transformer.py`
- [x] `items_transformer.py`
- [x] `user_transformer.py`
- [x] `batch_poster.py`