TST: restore type checks to maybe_promote tests
xref #25425.  Working on fixing the xfailed tests in this file.  This is going to be a marathon.
Very cool - I didn't expect that the typecheck could be re-introduced so easily. Thanks for tackling this!

OTOH, most of worst tests of #23982 were turned into dummies for #25637. [As I](https://github.com/pandas-dev/pandas/pull/25637#issuecomment-504488421) [offered](https://github.com/pandas-dev/pandas/pull/25425#pullrequestreview-255888873), I can rebase #23982, so we have the same baseline of tests available (somewhere *outside* #25424). Closing in favor of #28564.