TST: fix 24 xfails in maybe_promote
Vague gameplan: I think I've got a handle on the `if not boxed` xfail tests that are left, will incrementally fix them over a few more steps.  Then the `if boxed` cases all correspond to passing `ndarray` for `fill_value`, which never happens in the tests except for dedicated tests designed to do exactly that.  If we can rule that case out, we can remove most of the remaining xfails (and a lot of complexity in the test code) in one swoop.
Edit in internals.concat turns out to be necessary to fix a different test that the maybe_promote edit broke.  The relevant code in internals.concat should really use maybe_promote or something similar.  Will open an issue to that effect. thanks @jbrockmendel (hope these were orthogonal!) > hope these were orthogonal

Yep, I'm being pretty careful to keep these orthogonal so we can keep a tight turnaround on these.