platform/chrome: cros_ec_proto: add Kunit tests for get_next_event

cros_ec_get_next_event() gets events from EC.  It consists of 3
versions of event retrieval:

1. No MKBP event.
2. MKBP event version 0.
3. MKBP event version >0.

Add Kunit tests for cros_ec_get_next_event().

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220622041040.202737-4-tzungbi@kernel.org