TST: tests.groupby needs to be a package to run tests
skip parts of cummin test, xref #15109

seems the tests.groupby dir was being *skipped* :<

cc @mroeschke now #15109 is relevant you can just about see this, our codecov when from 85.3 to 84.5 when groupby tests dropped out :< https://codecov.io/gh/pandas-dev/pandas