Well, they don't crash them into each other but there are testing programs that crash them into standard objects like concrete walls. Crashing the new model into the old won't tell you much. Vehicles generally get larger/heavier with each new model. All you would probably learn is that newer Miatas weight more than old ones. Some quick googling shows that modern Miatas are about 10% heavier than they were in the 90s, probably due to the increase to 1.8 from 1.6 liter engines.