gh-84753: Make inspect.iscoroutinefunction() work with AsyncMock (#94050)

The inspect version was not working with unittest.mock.AsyncMock.

The fix introduces special-casing of AsyncMock in
`inspect.iscoroutinefunction` equivalent to the one
performed in `asyncio.iscoroutinefunction`.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>