First, although dinosaurs might have de facto become a test of faith for people, there is certainly no innate reason for them to be a test; as we shall see, there are plenty of ways in which the age of the dinosaurs is easily understood in the light of the Torah. (Challenge of Creation, page 159)This is curious. If he really felt this way one would suspect the book might be called "The Non-Challenge of Creation" or "The Un-Challenge of Creation". My apologies if that sounds too sarcastic, I do not mean to poke fun but instead demonstrate that this is a real challenge. The problem is that Rabbi Slifkin is one of many who simply do not appreciate the philosophical and theological tenuousness of interpretations that negate the plain meaning. It should be the last option, and even then requires explaining why this makes better since than to reject the text outright.
Now, I do not think that apparent age is intended as a "test" per se. At least not as a test independent of the "test" that is the physical world. Yes, Nature points to God, as Rabbi Slifkin points out. But it also conceals Him. This isn't debatable in a theistic context. This is the stuff of free will. God run's the world through the laws of nature, which sources including the Rambam indicate were not entirely set in stone until the completion of creation. At the same time it seems self obvious that the particulars of how God decided to run this world have their own reasons and if the natural order has apparent conflict with the miracle of creation, I'm not so certain we should find this surprising.