China and North Korea's relationship the last several years can be described as strained. Further, China is emerging as one of the strongest economies in the world, with the Untied States being it's principle trading partner. Most likely, were the U.S. to shoot down the test missle, China would make some noise but not much else.
They know a war in Korea would bring immediate American involvment, and that the U.S. has said it would be willing to us tactical nuclear weapons in the defense of South Korea (we've said so for decades). Further, North Korea's famine has decimated it's population and they are dependent on American grain shipments. They likely could not mount an extended war in the south without heavy Chinese support. This support would be used to directly attack U.S. troops, and most relations with China would collapse. Any nuclear exchange would result is far more damage to them then to America, so a drawn out conventioanl war is what they would want (to give the American people time to get angry and demad troop pullouts as the body count rises).
While future Chinese aggresion is a rising concern, I doubt they would support a Korean war at this time. Were they ever, it would probably be done alongside an assualt on Taiwan, which they care a whole lot more about fighting over. It would also mean risking a unilateral war with the world's strongest nation (economically and militarily, while they would technically have N. Korea, they effectiveness of that alliance is questionable). In the last war, they had the Soviet Union standing behind (and flying above) them. Were they to go to war in a place where we had troops, there would have to be a tangible gain to them. As it is, the region is at peace (except for N. Korea's sabre rattleing), they have no desire to change that unless they feel they have to (its bad for buisness).
Everyone knows that the North Koreans have no purpose in amasing ballistic missles, as the U.S. has no intention of intiating a war over there, and the only practical use stratigic missles have are either as a deterant or as a blitz tactic (and they can't afford an arms build up of that scale). The only reason they want them, is so that they can beat thier chest, and pretend they are important. China knows that, I doubt they are willing to risk a war just because Kim Jong's ego got bruised.