tests: drivers: ipm: Fix drivers.ipc.mailbox

Fix 'drivers.ipc.mailbox' test suite to restore and improve
its testing functionality:

 * fix printf() redirection through IPM console.

 * fix and check the dummy driver initialization.

 * specify correct Twister harness type (Console)
   and use matching patterns to recognize the expected
   console output.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>