bpo-38356: Fix ThreadedChildWatcher thread leak in test_asyncio (GH-16552)

Motivation for this PR (comment from @vstinner in bpo issue):
```
Warning seen o AMD64 Ubuntu Shared 3.x buildbot:
https://buildbot.python.org/all/#/builders/141/builds/2593

test_devnull_output (test.test_a=syncio.test_subprocess.SubprocessThreadedWatcherTests) ...
Warning -- threading_cleanup() failed to cleanup 1 threads (count: 1, dangling: 2)
```
The following implementation details for the new method are TBD:

1) Public vs private

2) Inclusion in `close()`

3) Name

4) Coroutine vs subroutine method

5) *timeout* parameter

If it's a private method, 3, 4, and 5 are significantly less important.

I started with the most minimal implementation that fixes the dangling threads without modifying the regression tests, which I think is particularly important. I typically try to avoid directly modifying existing tests as much as possible unless it's necessary to do so. However, I am open to changing any part of this.


https://bugs.python.org/issue38356

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