It would be a cool toy, but I can just imagine the results if such a tester became common. You'd spend the rest of your life wading through threads titled, "Is a 2.3% mismatch in gain too much to run in my Boutique-O-Drive Gain Master 60™?"
I can't say its necessarily the right approach, but I tend to think of tubes like guitar strings... put them in, work the "stretch" out of them and tune them up, then run them until they sound bad.