Extend test cases for flake8-pyi

[sbrugman]

Summary

This PR extends the test cases for a couple of flake8-pyi rules with nested and mixed union annotations.
Although these are rare, they are valid:

>>> from typing import Union
>>> Union[int | int | float]
typing.Union[int, float]

>>> Union[Union[float, complex]]
typing.Union[float, complex]

We should be able to handle them anyhow, as users or fixes might write these accidentally.

Test Plan

The snapshots are updated with the as-is behaviour.
I'll follow up with a change to the nested union logic. Having these tests in place before makes it easier to review the diff.

[AlexWaygood]

This has merge conflicts; could you take a look?

[sbrugman]

I'll do it in one go after #14270, #14272 and #14273 are merged to avoid duplicate work.