Clean up references to threads in test_queue.

Lib/test/test_queue.py

@@ -43,6 +43,9 @@ class _TriggerThread(threading.Thread):
 
 class BlockingTestMixin:
 
+    def tearDown(self):
+        self.t = None
+
     def do_blocking_test(self, block_func, block_args, trigger_func, trigger_args):
         self.t = _TriggerThread(trigger_func, trigger_args)
         self.t.start()
@@ -222,7 +225,7 @@ class FailingQueue(Queue.Queue):
             raise FailingQueueException, "You Lose"
         return Queue.Queue._get(self)
 
-class FailingQueueTest(unittest.TestCase, BlockingTestMixin):
+class FailingQueueTest(BlockingTestMixin, unittest.TestCase):
 
     def failing_queue_test(self, q):
         if not q.empty():