Add capability to test for `Never`

[loicdiridollou]

Currently it is a bit clunky to test for the return type Never, it would be best to condense it into a check_never or alike so we can clean up the code around the tests folder.

[cmp0xff]

Never can be hard to catch, consider the following:

from typing import TYPE_CHECKING
from typing_extensions import Never


def gives_never() -> Never:
    raise RuntimeError


def catch_never(never: Never) -> None:
    return


def should_fail(p: int) -> None:
    return


if TYPE_CHECKING:
    catch_never(gives_never())

if TYPE_CHECKING:
    should_fail("1")  # neither pyright nor mypy complains, both ty and pyrefly complain

[jorenham]

Note that Any is also assignable to Never, and that there's no way to "exclude" it as far as I'm aware.