I’m honestly not sure how one might contribute to the official test suites these days. I don’t even necessarily how the suites work, now. It used to be, you just put up a bunch of pages and people would look at them to determine if a browser passed.
That said, I’m a big proponent of building test cases, even if they’re just for your own understanding. I maintain a set of test pages at http://meyerweb.com/eric/css/tests/css3/ and another collection one directory up, at http://meyerweb.com/eric/css/tests/ — they’re not meant to be a formal suite, but they help me work out how things are supposed to work.