Added test test_datetimeField_after_setitem for issue #6942
Added test `test_datetimeField_after_setitem` in `tests/generic/test_frame.py`.

- [x] closes #6942 
- [x] test added
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
