I would rather they release the test track now instead of waiting a year. If they did that, they might be able to squeeze a little more interest out of FM3 and pull away some casual fans from Sony who are just interested in GT5 for Top Gear's presence. And I think a lot of people would buy it, actually. I haven't played FM3 in months, but I'd definitely get back into it if it means doing some time trials on the test track.
It kind of comes off as half-assed to do it this way though. If you want to provide the "definitive Top Gear experience" or whatever in FM4, wait till FM4 to get the ball rolling. Because a three car pack and what is likely to be a 90 minute clip show aren't going to help make a year old game with little developer support and a dwindling number of players online relevant again.