make sure iopub thread is started in test

stop closes the event puller socket, but only if start was called

Author: minrk

ipykernel/tests/test_io.py

@@ -123,6 +123,7 @@ def subprocess_test_echo_watch():
     with zmq.Context() as ctx, ctx.socket(zmq.PUSH) as pub:
         pub.connect(os.environ["IOPUB_URL"])
         iopub_thread = IOPubThread(pub)
+        iopub_thread.start()
         stdout_fd = sys.stdout.fileno()
         sys.stdout.flush()
         stream = OutStream(