TST/REF: loc/iloc/at/iat tests go in tests.indexing
not tests.(series|frame).indexing

`(Series|DataFrame).__(getitem|setitem)__` tests go in the tests.(series|frame).indexing


sure I would add a README.me i think to tests/frame & tests/series to indicate the test structure a bit more (e.g. the methods/ dir) and the fact that all indexing tests go in test/indexing (alt we can have a top-level readme do this). yah ive got a half-written file to that effect