multiplexed_socket: add ChannelSocketTest

The usage of ChannelSocketTest helps avoiding unnecessary overhead when
running unit tests (especially on creating a huge quantity of nodes and
sockets).

ChannelSocketTest implements a simplier form of ChannelSocket that
directly communicates to a peer rather than through a socket.

Change-Id: Id1c68aaa92f8f8cf8002c417f670254b0b851cfb