chore: Add test for async foreign function

[langenhagen]

I pondered whether or and how to consolidate the foreign function tests best and decided that mixing the other foreign function assertions with the original test_run for a certain ... entropy and adding a new test for this async function is a good middle ground in order to keep the tests all bearably simple.

Ahja fwiw, ftr:

>>> import asyncio
>>> async def foo() -> int:
...     await asyncio.sleep(0.1)
...     return 42
...
>>> type(foo)
<class 'function'>
>>> def bar():
...     return 42
...
>>> type(bar)
<class 'function'>
>>> from typing import Callable
>>> isinstance(foo,Callable)
True
>>> isinstance(bar,Callable)
True
>>>