test: Close a websocket client that causes occasional test failures

These failures occur on the build.yml workflow on the emulated arm64
platform: an ill-timed timer firing during test shutdown can result
in a message being logged and the test failing for dirty logs.

tornado/test/websocket_test.py

@@ -807,6 +807,7 @@ def test_client_ping(self):
             response = yield ws.read_message()
             self.assertEqual(response, "got ping")
         # TODO: test that the connection gets closed if ping responses stop.
+        ws.close()
 
 
 class ManualPingTest(WebSocketBaseTestCase):