TST: Fix maybe_promote xfails
Fix a handful of these, clean up a bunch of others.

This gets towards the end of what I can clean up without major changes.  I'm inclined to try to remove the `box=True` cases and disallow ndarray `fill_value` arg, which would simplify the code a bit and the tests a ton.  AFAICT we can do this without breaking any existing tests (outside of tests for `dtypes.cast`), but I don't think we can rule out it being _possible_ to get there in the handful of places where maybe_promote is called. Reverted unnecessary test cleanup to make a smaller diff maybe rebase this as we merged a few other things, ping on green. sure.  heads up: im out of town until monday, will be less responsive than usual ping thanks