[3.6] bpo-31234: test_httpservers joins the server thread (GH-3188) (#3536)

(cherry picked from commit 830d7d29)

Lib/test/test_httpservers.py

@@ -52,6 +52,7 @@ class TestServerThread(threading.Thread):
 
     def stop(self):
         self.server.shutdown()
+        self.join()
 
 
 class BaseTestCase(unittest.TestCase):