samples: CAN: Add testing harness

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

We expect the sample to report back somthing like:

Counter received: 1
Counter received: 2
Counter received: 3
Counter received: 4
...

etc.

So have a simple regex check for it.

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