gh-129158: Ensure we restore unix_events.can_use_pidfd after SubprocessThreadedWatcherTests finishes (#129160)

Author: pablogsal

Lib/test/test_asyncio/test_subprocess.py

@@ -901,9 +901,14 @@ def test_watcher_implementation(self):
     class SubprocessThreadedWatcherTests(SubprocessWatcherMixin,
                                          test_utils.TestCase):
         def setUp(self):
+            self._original_can_use_pidfd = unix_events.can_use_pidfd
             # Force the use of the threaded child watcher
             unix_events.can_use_pidfd = mock.Mock(return_value=False)
             super().setUp()
+        
+        def tearDown(self):
+            unix_events.can_use_pidfd = self._original_can_use_pidfd
+            return super().tearDown()
 
     @unittest.skipUnless(
         unix_events.can_use_pidfd(),