need unit test that ssh_CHANNEL_OPEN sends a packet rather than raising a TypeError

Description

Prior to the changes in #3657, ConchUser.lookupChannel raised a ConchError with the arguments reversed. twisted.conch.ssh.connection.ssh_CHANNEL_OPEN uses the attributes of ConchError to send an SSH message so this reversal caused the traceback described at the top of #3657.

Once #3657 is closed, we need a unit test that the behavior is now correct and ssh_CHANNEL_OPEN sends a packet rather than raising a TypeError.

The test looks good, as far as it goes. It needs to go a bit futher, though (and its docstring suggests this): ssh_CHANNEL_OPEN ends with a call to sendPacket; the test should verify that that call is doing the right thing. I'm not quite sure what the right thing is, or whether the implementation is currently correct or not, partly because I'm not sure how this patch is intended to interact with the patch from #3657 (and partly because this is a pretty low-level part of Conch which I haven't looked at before :). Can you elaborate on that, or re-submit this after #3657 is resolved?