Added two tests for issue #29697
I added two basic tests for `merge` as requested in #29697, one for `how="outer"` and one for  `how="right"`. 

I'm not sure if the way I'm creating duplicate columns for the `expected` dataframe is the recommended way but I could not find anything on this. Please let me know if there's a better way to do this.

- [x] closes #29697
- [x] 2 tests added / 2 passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`



