From last year, the gates opened at 12:30 to let the public in and the test starts at 1 and runs till 5.
The pit lane was open to walk up and down before the test started and even during you could stand in the pit lane and get on the pit wall to watch the action, just make sure you don't get run over.
Here's my vid from last year.
https://www.youtube.com/watch?v=LVpQFN09_VQ