Changed bits of test to handle checking of sent messages.

Should have an incomplete library, preventing a complete build.

Signed-off-by: Jacky Alciné <me@jalcine.me>
Added source files for tunnel code (mostly empty).
Updated tests for tunnel so that it won't be built.

  In order to ensure that the tunnel test can't be built, it's been
  hardwired to rely on the declaration of it, the Dispatcher and the
  Receiver. Enough for one night.