TST: reorg groupby tests

[jreback]

see suggestions here: #12607 (comment)

we have lots and lots of groupby tests and so getting pretty unweidly

so should reorg to a sub-dir setup like we have for other things

eg.

pandas/tests/groupby/.....

[aileronajay]

@jreback so this change would involve relocating pandas/tests/test_groupby.py to the hierarchy pandas/tests/groupby/ and then breaking this file into other files (having tests of similar type)?

[jreback]

yes

[aileronajay]

I have created this PR for the change, https://github.com/pandas-dev/pandas/pull/14817/commits