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fix the nc wait. the issue was that stdin needs to remain _open_
but not receive any input for some time. If stdin receives (invalid)
input or closes, the other side terminates the connection before
writing out the muxer frames + identify handshake.

This commit also changes the use of `!` for `test_must_fail`

License: MIT
Signed-off-by: Juan Batiz-Benet <juan@benet.ai>