Kill a small potential leak in test_threading.

The leak may not manifest itself if the OS re-uses the same thread ids
(I suppose Neal's machine doesn't :-))

Lib/test/test_threading.py

@@ -111,6 +111,8 @@ class ThreadTests(BaseTestCase):
         thread.start_new_thread(f, ())
         done.wait()
         self.assertFalse(ident[0] is None)
+        # Kill the "immortal" _DummyThread
+        del threading._active[ident[0]]
 
     # run with a small(ish) thread stack size (256kB)
     def test_various_ops_small_stack(self):