CLN: Group tests for apply/agg/transform with a string argument
- [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

Reorg of apply tests, grouping all those that take a string argument together. Plan to next create `test_list` and `test_dict`, then all other test files in apply will have (at least partially) UDFs, which make up a majority of the tests.

Had to rename a few tests because of collisions, but otherwise purely a move.
@jreback - greenish