platform/chrome: kunit: make EC protocol tests independent

Remove CONFIG_CROS_KUNIT and common code concept for ChromeOS Kunit but
make it bundle to ChromeOS EC protocol tests.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20231003080515.4011374-1-tzungbi@kernel.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>