TST: Add testing to df.where() typecasting as per GH 42295
- [x] closes #42295
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

This makes sure that df.where does not cast floats to integers.
Hello @maximilianaccardo! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-08-26 01:52:01 UTC Thanks @maximilianaccardo. Happy to have you tackle more issues that require tests. 