cros_ec_get_sensor_count() gets number of MEMS sensors.

Add Kunit tests for cros_ec_get_sensor_count().
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220622041040.202737-7-tzungbi@kernel.org