platform/chrome: cros_ec_proto: add Kunit tests for get_host_event

cros_ec_get_host_event() performs some sanity checks, parses
`ec_dev->event_data.data.host_event`, and returns bitmap of
EC_HOST_EVENT_*.

Add Kunit tests for cros_ec_get_host_event().

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220622041040.202737-5-tzungbi@kernel.org