platform/chrome: merge Kunit utils and test cases

Merge CROS_KUNIT and CROS_EC_PROTO_KUNIT_TEST so that when they're built
as modules cros_kunit_util doesn't need to export the symbols.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220720044754.4026295-2-tzungbi@kernel.org