There's always been a lot of speculation about the Test Track and why it wasn't included in the game at launch or if ever.
Now, I don't want to rain on anyone's parade but looking at it now... I don't think I'd want it as DLC for $$$ or as DLC at all. Think about this... Test Track was mostly known for it's role in Machine Test, which in GT5 is completely absent from the game. It was also a track in the Like The Wind series, but has since been replaced by Indy (I'm assuming).
Now in GT5 there are two high speed oval tracks, neither one you can get much speed out of in the straights, and then there is SSR7, which is what I'm most suspicious about replacing Test Track possibly in GT6 as well. And I do think it has
something to do with license agreements with certain manufacturers, as someone pointed out in this thread eg. Ferrari. I have come to realize out of all the tracks in the game, there is no place to give hard 0-60, 0-400, 0-1000, or even
true top speeds. For example, my top speeds are different in Sarthe (usually greater) than in SSR7 and Nurb. There doesn't seem to be a neutral ground for this sort of thing, only tracks which just happen to have long straights over different types of terrain which make differences in top speed testing. Usually Machine Test and Test Track would be the neutral ground.
But anyway... I don't really see Test Track being something worth money, but if it were a freebie, I guess I'd take it. It would also be weird to have the Test Track but then no Machine Test that sets up the 400M/1,000M/Top Speed tests for us. I mean it isn't a particuarly interesting track itself - I guess the drafting battles would be fun online, but really we have tracks for that already. I just don't see the appeal in it without a Machine Test. And unless Ferrari is going to send out a team of people to tune your car before every test, I don't think we'll see it. If it were going to be in the game, it would've been here at launch like every other title before it (except GT5

).
Just IMO. I don't have hard facts, just some speculation. I will note that GT

SP had Test Course with Ferrari, but that game really is a completely different beast from GT5.