platform/chrome: cros_ec_proto: add Kunit test for cros_ec_cmd()

cros_ec_cmd() is a wrapper of cros_ec_cmd_xfer_status().

Add Kunit test for cros_ec_cmd().

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220622041040.202737-8-tzungbi@kernel.org