Groupby tests restructure

closes pandas-dev#14813

Author: Ajay Saxena <[email protected]>

Closes pandas-dev#14817 from aileronajay/groupby_test_restructure and squashes the following commits:

860574d [Ajay Saxena] removed duplicate file
f6e1cda [Ajay Saxena] further split the tests
2cc0734 [Ajay Saxena] branched out the filter tests into a new file

Author: aileronajay