You can head to the Test Track at any time - you only need a car that is complete and working. By driving around the designed track and stopping in the right place, you can learn a lot about the car's performance. Besides, it's cool to just try out the car you made yourself.
If you're looking for some entertainment, you should like the Race Track more. You can take any car you have that's complete and working, and push it on the track. The lap time is saved, counted by checkpoints - you can cheat by cutting some corners.