The test was verifying that when a DISCONNECT frame is sent to the
broker, there will be no further messages on the clientOutboundChannel.
This is generally true, however in some cases when the broker receives
a DISCONNECT it may close its connection fast enough (before we do) in
which case we send an ERROR message downstream to ensure the WebSocket
side is cleaned up. Either way the downstream should be idempotent
with regards to cleaning up sessions.
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