tests/boards/intel_adsp: Add ipm_cavs_host test

Add a fairly simple test of the IPM-over-IPC driver.  This hits all
the code, but works by implementing the host side of the protocol
partially in the C test code.  The message is sent with an initial
payload, and then IPC commands from the firmware copy the data over
into the "inbox" region to simulate data being sent via the host.
Then we make sure it lands correctly as if the host driver had done it
directly.

This requries a new command in the cavstool script that will copy a
word from the "outbox" region to the "inbox" region (both are just
different SRAM windows, conceptually no different than the way the
script is already managing log output), but no significant surgery.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>