test: Add initial tests for mi-mctp layer

test: Add initial tests for mi-mctp layer

This change adds a new test module, for the MCTP transport of the MI
layer.

To do this, we hook into the socket, sendmsg and recvmsg calls from the
core mi-mctp code, allowing the test to provide a mock communication
with the kernel MCTP layer.

On top of this, we write a few simple test cases for the socket error
resonses, which we'll extend in future changes.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>