TST: astyp via loc to int64
- [x] closes #31861
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Adds a test for a special case of `astype` via `loc` to extension type Int64 when data has np.nan or pd.NA. 