Test random failures

[SylvainCorlay]

Occasionally, we see one of the builds in CI fail with errors such as the following: (see e.g. https://github.com/ipython/ipykernel/runs/2173713500)

Details

_________________________________ test_execute _________________________________

    def test_execute():
        flush_channels()
    
        msg_id = KC.execute(code='x=1')
>       reply = get_reply(KC, msg_id, TIMEOUT)

ipykernel/tests/test_message_spec.py:282: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
ipykernel/tests/utils.py:60: in get_reply
    reply = kc.get_shell_msg(timeout=timeout)
/opt/hostedtoolcache/Python/3.9.2/x64/lib/python3.9/site-packages/jupyter_client/client.py:78: in get_shell_msg
    return self.shell_channel.get_msg(*args, **kwargs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <jupyter_client.blocking.channels.ZMQSocketChannel object at 0x7fcb0541c610>
block = True, timeout = 15000

    def get_msg(self, block=True, timeout=None):
        """ Gets a message if there is one that is ready. """
        if block:
            if timeout is not None:
                timeout *= 1000  # seconds to ms
            ready = self.socket.poll(timeout)
        else:
            ready = self.socket.poll(timeout=0)
    
        if ready:
            return self._recv()
        else:
>           raise Empty
E           _queue.Empty

/opt/hostedtoolcache/Python/3.9.2/x64/lib/python3.9/site-packages/jupyter_client/blocking/channels.py:54: Empty

However, I cannot reproduce that issue locally. Interestingly, in these builds, we see these errors for multiple tests, which seem to indicate that some common initialization thing is not working properly. We also see these failures in jupyter_client's test when testing with the pre-release of ipykernel.

[blink1073]

Fixed by #615.