platform/chrome: cros_ec_proto: add Kunit test for getting legacy info

cros_ec_get_proto_info_legacy() expects to receive
sizeof(struct ec_response_hello) from send_command().  The payload is
valid only if the return value is positive.

Add a Kunit test for returning 0 from send_command() in
cros_ec_get_proto_info_legacy().

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-12-tzungbi@kernel.org