platform/chrome: cros_ec_proto: add Kunit test for cros_ec_map_error()

cros_ec_cmd_xfer_status() is the only exported function that calls
static function cros_ec_map_error().

Add Kunit test for cros_ec_map_error() through calling
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/20220622041040.202737-3-tzungbi@kernel.org