[3.14] gh-139894: fix incorrect sharing of current task while forking in `asyncio...

[3.14] gh-139894: fix incorrect sharing of current task while forking in `asyncio` (GH-139897) (#139913)

* [3.14] gh-139894: fix incorrect sharing of current task while forking in `asyncio`  (GH-139897)

Fix incorrect sharing of current task with the forked child process by clearing thread state's current task and current loop in `PyOS_AfterFork_Child`.
(cherry picked from commit b881df47ff1adca515d1de04f689160ddae72142)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
* Update Lib/test/test_asyncio/test_unix_events.py