I actually believe that Coriakin could be the villain. Not because of the lines in the trailer, but because of a line in the book here:
"In this world you have already met a star: for I think you have been with Coriakin."
"Is he a retired star, too?" said Lucy.
"Well, not quite the same," said Ramandu. "It was not quite as a rest that he was set to govern the Duffers. You might call it a punishment. He might have shone for thousands of years more in the southern winter sky if all had gone well."
Because of this line in the book, I can easily see the director and writers turning Coriakin into a villain, or at least making him out to be a harsher character then one to have "an open, ongoing invitation for tea [with Aslan]."
One of the things that has made me wonder the most about how a movie adaptation of VDT would be, in comparison with other Narnia movies, is that VDT is the most unique book in the series, not focusing on the universal plot as much as going from one event to the other. An example of this is how each problem ours heroes face- being kidnapped by slave owners, being turning into dragons- is solved before the chapter ends. In comparison, the other books has a constant buildup to the very end.
Coriakin, being a central villain, could be expanded and create another universal plot. Let me be clear, I know that the main plot in VDT is to find the seven missing lords. But again, as they are found almost completely one-by-one, VDT is given a step-by-step feel as a pose to a buildup story- which is something that I believe most movie workers would shoot for.
Of course, when looking at the the trailer, and all the other movie news we have, there is no reason to suggest this, for, as others have pointed out, "You are about to be tested" could be uttered by ether a villain or hero character. But I do believe that Coriakin and Lilliandil will be more important in the movie than the book, no matter how different their characters are. It would seem to me, that if this was the case, then Ramandu would also have a bigger part, but as nobody has been cast for his part, there could be even more changes to Coriakin and Lilliandil.
I'm not saying that I believe Coriakin will be a villain, but I do think that the possibility is still there. I will not assume anything, only wonder what could be, and I will also reserve my judgments on whether-or-not this is a good thing, until I see the movie.
I'm sorry for writing so much, and for not checking grammar, this was suppose to be a quick thought, but sometimes my mind gets ahead of me.