pythongh-130730: Fix flaky test_active_children test

Use an Event object instead of time.sleep().

Author: colesbury

Lib/test/_test_multiprocessing.py

@@ -589,13 +589,15 @@ def test_cpu_count(self):
     def test_active_children(self):
         self.assertEqual(type(self.active_children()), list)
 
-        p = self.Process(target=time.sleep, args=(DELTA,))
+        event = self.Event()
+        p = self.Process(target=event.wait)
         self.assertNotIn(p, self.active_children())
 
         p.daemon = True
         p.start()
         self.assertIn(p, self.active_children())
 
+        event.set()
         p.join()
         self.assertNotIn(p, self.active_children())