samples: ipc: openamp: Add testing harness

Add console testing harness for expected output.  This allows us
to validate that the test is running properly on hardware.

We expect output of the form:

Master core received a message: 1
Master core received a message: 3
Master core received a message: 5
    ...
Master core received a message: 95
Master core received a message: 97
Master core received a message: 99
OpenAMP demo ended.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>