We like Soarin a lot, so we usually like to ride it twice on a visit. If that is something you're interested in doing, my advice would be to arrive by park opening and immediately grab a set of Soarin fastpasses. Then ride it in the standby line. Go ride Test Track next and come back for your second Soarin ride. Lot of extra walking in that plan though!
If only planning to ride it once, I don't think the question of which to ride first is critical. If it was me, I'd probably ride Test Track first, hop over and grab fastpasses for Soarin, then head to the Seas for Turtle Talk. Just minimizing backtracking by picking Test Track first.