TST: Fix maybe_promote xfails

[jbrockmendel]

Fix a handful of these, clean up a bunch of others.

[jbrockmendel]

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.

[jbrockmendel]

Reverted unnecessary test cleanup to make a smaller diff

[jreback]

maybe rebase this as we merged a few other things, ping on green.

[jbrockmendel]

sure. heads up: im out of town until monday, will be less responsive than usual

[jbrockmendel]

ping

[jreback]

thanks