WIP/TST: add dt64tz to indices fixture
@TomAugspurger @jorisvandenbossche as you can see, the `__array_ufunc__` that this implements is an unholy mess.  This all came from a much smaller goal: I wanted to add the "datetime-tz" key in tests.indexes.conftest.

That broke a test for `np.isfinite`, so I started implementing `array_ufunc`.  Then that broke other ufuncs, and so on and so on.

I guess I could just xfail the one isfinite test.  In retrospect that would have saved a lot of effort.  Woops.

Anyhow, we probably want/need to implement this eventually anyway, so any ideas on how to do it but less awful?

xref #31219.
Closing in favor of #31241