Compare cwd() of Popen Gateways
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A PopenGateway has the same working directory as the instantiatior::

    >>> import execnet, os
    >>> gw = execnet.PopenGateway()
    >>> ch = gw.remote_exec("import os; channel.send(os.getcwd())")
    >>> res = ch.receive()
    >>> assert res == os.getcwd()
