CLN: Split and fixturized test_fillna in tests/base/test_ops.py
part of #23877
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

@MomIsBestFriend Seems like that didn't work for the `isinstance(obj, ABCIndex)` cases because...
```
isinstance(pd.Float64Index([]), ABCIndex) is False
isinstance(pd.IntervalIndex([]), ABCIndex) is False
isinstance(pd.CategoricalIndex([]), ABCIndex) is False
...
```

I'll revert it to a simple `isinstance(obj, Index)` thanks @SaturnFromTitan 

ideally if you can add the nulls fixture would be great :->