As it is a test event, I'm sure they'll be monitoring how many people are using public transport and how the system can cope.
There are parking spaces at Westfield. The Olympics are supposed to be "Car Free" as it is environmentally friendly, to use public transportation. Every Olympic ticket is coming with a zone 1 - 6 travel card. I don't know if your test event tickets come with that or not.
The Olympic park has NO PARKING, then therefore, the Olympic Park has no Car Parking. If you want to use Westfield, then that is your choice, but if all 6000 people who are attending brought 1 car, there would be 1000 people at the test event, without a parking space at Westfield, and also, no one who wanted to shop, to go shopping wouldn't be able to park. Also, there wouldn't be a stress test on the Public Transportation.
Also, if I can't get a parking space when I want a parking space, because you wanted to drive, to an olympic test event, when I want to go shopping in Waitrose, or M&S, because they're convenient for me, then I may be rather annoyed. Think about the people who live in Stratford, and how you could be inconveniencing them, (me), by bringing your car.
M