platform/chrome: cros_ec_proto: add Kunit test for empty payload

cros_ec_wait_until_complete() sends EC_CMD_GET_COMMS_STATUS which expects
to receive sizeof(struct ec_response_get_comms_status) from
cros_ec_xfer_command().

Add Kunit test and expect to receive an error code when
cros_ec_xfer_command() returns 0.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220718050914.2267370-10-tzungbi@kernel.org