platform/chrome: cros_ec_proto: add Kunit test for getting wake mask

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

Add Kunit tests for returning 0 from send_command() in
cros_ec_get_host_event_wake_mask().

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-21-tzungbi@kernel.org