platform/chrome: cros_ec_proto: add Kunit tests for cmd_xfer_status

cros_ec_cmd_xfer_status() calls cros_ec_cmd_xfer() and cros_ec_map_error().

Given that there are already test cases for cros_ec_cmd_xfer(), only add
basic Kunit tests for cros_ec_cmd_xfer_status().

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220622041040.202737-2-tzungbi@kernel.org