gh-110205: Fix asyncio ThreadedChildWatcher._join_threads() (#110884)

- `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`.

Lib/test/test_asyncio/utils.py

@@ -546,6 +546,7 @@ class TestCase(unittest.TestCase):
             else:
                 loop._default_executor.shutdown(wait=True)
         loop.close()
+
         policy = support.maybe_get_event_loop_policy()
         if policy is not None:
             try:
@@ -557,9 +558,13 @@ class TestCase(unittest.TestCase):
                 pass
             else:
                 if isinstance(watcher, asyncio.ThreadedChildWatcher):
-                    threads = list(watcher._threads.values())
-                    for thread in threads:
-                        thread.join()
+                    # Wait for subprocess to finish, but not forever
+                    for thread in list(watcher._threads.values()):
+                        thread.join(timeout=support.SHORT_TIMEOUT)
+                        if thread.is_alive():
+                            raise RuntimeError(f"thread {thread} still alive: "
+                                               "subprocess still running")
+
 
     def set_event_loop(self, loop, *, cleanup=True):
         if loop is None: