stubtest: basic support for unpack kwargs - #21024

Fixes #22238

#21024 made stubtest expand `**kwargs: Unpack[TypedDict]` into keyword
arguments, but only in `Signature.from_funcitem`. Overloads go through
`from_overloadedfuncdef`, which reads each item's raw arguments, so they
still got a plain `**kwargs` and stubtest reported every key as missing.

This moves the expansion into `expand_unpacked_kwargs()` and applies it
to plain functions and to each overload item. The merged overload
signature then picks up the keys like any other keyword arguments, so a
key only present in some overloads is treated as optional.