CLN/TST: clean logic of old datetime test_indexing test
Breaking off a change I had to make for https://github.com/pandas-dev/pandas/pull/55428, because it's actually unrelated to CoW and reduces the diff there a little bit.

I think that this test has over time, through several small changes, become quite useful / incomprehensible. So cleaning it up, and actually asserting what my understanding is that it was originally testing.
Thanks @jorisvandenbossche 