[3.12] gh-110205: Fix asyncio ThreadedChildWatcher._join_threads() (GH-110884) (#111412)

- `ThreadedChildWatcher.close()` is now *officially* a no-op; `_join_threads()` never did anything.
- Threads created by that class are now named `asyncio-waitpid-NNN`.
- `test.test_asyncio.utils.TestCase.close_loop()` now waits for the child watcher's threads, but not forever; if a thread hangs, it raises `RuntimeError`.
(cherry picked from commit c3bb10c9303503e7b55a7bdf9acfa6b3bcb699c6)

Co-authored-by: Guido van Rossum <guido@python.org>