The stickies say its closed, as it has for the last 10 days or so.
Whether you can join the test sessions on Thursday comes down to whether there is room in there after the signed up people are all there, its first come first served for the signed up folk, 32 slots, 50 signed up, so you'll only get in assuming a significant number of people dont show up which is reasonably likely based on the last few test dates but as this is the last one before the series and will probably be run like a proper round it may be busier than usual.
Turn up if your available, but you'd only get in if theres room after registered folk are in.