platform/chrome: cros_ec_proto: add Kunit tests for cros_ec_cmd_xfer()

cros_ec_cmd_xfer() transfers the given command and data if any.  It
performs some sanity checks and calls cros_ec_send_command().

Add Kunit tests for cros_ec_cmd_xfer().

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220718050914.2267370-4-tzungbi@kernel.org