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.