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

cros_ec_prepare_tx() is used to fill the protocol headers according to
the requested protocol version.

Add Kunit tests cros_ec_prepare_tx() for each version.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220518091814.2028579-2-tzungbi@kernel.org