Add tests asserting that EAs shouldn't be coerced to ndarray
https://github.com/pandas-dev/pandas/pull/24204 introduced a pattern for ensuring that an extension array (like categorical) isn't implicitly converted to an ndarray as part of a normal pandas operation.

This issue is to discuss

1. How these kinds of tests could be better written (is monkeypatching the best choice?)
2. Additional places where we should be making these assertions

