REF/TST: Move Series/DataFrame apply tests to tests.apply
- [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

Ref: https://github.com/pandas-dev/pandas/pull/39212#issuecomment-763112628

Just a move. Will be following up with combining Series/DataFrame tests where appropriate.