Use SO_REUSEADDR in low-level socket tests.

tornado/test/simple_httpclient_test.py

@@ -138,6 +138,7 @@ def test_connect_timeout(self):
         port = get_unused_port()
 
         with closing(socket.socket()) as sock:
+            sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             sock.bind(('', port))
             self.http_client.fetch("http://localhost:%d/" % port,
                                    self.stop,