CRAS: update todo in cras_tests

Update a todo to reflect that our wav encode and decode crate does have
S24_LE support.

BUG=None
TEST=None

Change-Id: I7c54f1cb5869306d011a67720d4aeb43e0c0b83e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/2543299
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
Tested-by: Chih-Yang Hsia <paulhsia@chromium.org>
Commit-Queue: Fletcher Woodruff <fletcherw@chromium.org>