[red-knot] Test: Hashable/Sized => A/B (#14769)
## Summary
Minor change that uses two plain classes `A` and `B` instead of
`typing.Sized` and `typing.Hashable`.
The motivation is twofold: I remember that I was confused when I first
saw this test. Was there anything specific to `Sized` and `Hashable`
that was relevant here? (there is, these classes are not overlapping;
and you can build a proper intersection from them; but that's true for
almost all non-builtin classes).
I now ran into another problem while working on #14758: `Sized` and
`Hashable` are protocols that we don't fully understand yet. This
causing some trouble when trying to infer whether these are fully-static
types or not.
