CLN: Split test_fields.py
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them

@jbrockmendel https://github.com/pandas-dev/pandas/runs/4453493254?check_suite_focus=true maybe something changed in numpy? some of them look like changed exception messages which isn't too bad.  not sure about the reductions > some of them look like changed exception messages which isn't too bad. not sure about the reductions

great thanks for having a look @jbrockmendel 