TST/REF: collect DataFrame reduction tests
NB: this retains all extant fixture usage.

Collect tests by method being tested.  Collect all calls to assert_stat_op_api in one test (one test which can be usefully parametrized/fixturized in a follow-up), removing a ton of noise from other test function signatures.  As a result, it becomes much easier to see places where we can _usefully_ parametrize/fixturize tests in this module.  e.g. test_min and test_max are nearly identical.  ditto test_cumsum/test_cumprod/test_cummin/test_cummax.  ditto test_idxmin/test_idxmax...