tests: mbox_api: Fully initialize k_box_msg struct

`mmsg` should be fully initialized prior to calling k_mbox_get().  The
mailbox implementation (mainly, mbox_message_match()) will look inside
the second parameter passed to k_mbox_get() in order to determine if
which pending messages should be returned to the caller.

Fixes Valgrind warning listed in #7478.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>