TYPING: Added types for some tests
Working around a strange typing issue. See
https://github.com/pandas-dev/pandas/pull/28394#issuecomment-5456527290
for more, but the types on these were being inferred incorrectly by
mypy with just the addition of the `allows_duplicate_labels` kwarg.

cc @simonjayhawkins & @WillAyd. Hopefully I got things correct. I copied the use of TypeVar for FrameOrSeries with IndexOrSeries.
