gh-100980: ctypes: Test, document, and fix finalizing _fields_ (GH-124292)

- If setting `_fields_` fails, e.g. with AttributeError, don't set the attribute in `__dict__`
- Document the “finalization” behaviour
- Beef up tests: add `getattr`, test Union as well as Structure
- Put common functionality in a common function

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>