Add fillna_method 'pad' extension array tests
Should we also be adding `'backfill'`?

- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Since pad is an alias of ffill, adding this is almost redundant with what's already tested. 

Additionally we deprecated & removed similarly named methods (https://github.com/pandas-dev/pandas/pull/49081) as we're trying to eliminate aliases, so I would opt to even deprecate "pad"  Oops. I didn't realize it's just an alias.