The more you can get the aerobic system to contribute to the snatch test effort, the better off you'll be. The aerobic system takes a few minutes to ramp up, and it varies by individual. I look at it this way:
- If you have poor aerobic fitness, warm-up matters less. You'll be relying heavily on glycolysis anyway to get the test done. (This is entirely possible, but it's miserable and you'll feel smoked afterwards).
- If you have decent/average aerobic fitness AND you take a few minutes to "ramp up" your aerobic system (5-10 min of walking, a few minutes of light jogging in place, jumping jacks), it will contribute more to your effort. You'll perform better and feel better afterwards.
- If you have REALLY good aerobic fitenss, one feature of being aerobically fit is that your aerobic energy system "ramps up" quicker. So as little as 1-2 minutes of the above warm-up might do the trick. The snatch test will be easy, IF you're also strong and snatch well. If you're a skinny triathlete type, you'll still have a hard time with the muscular endurance and power portion, but your breathing and heart rate won't be your limiter.
I usually do 50 jumping jacks, maybe 2 min of jumprope, 10 min of walking... one of these, with another minute or two to settle back down before starting. Get the heart rate and breathing slightly elevated without bringing on any fatigue. I'll also make sure to do at least one set of 5 snatches with each hand a minute or two prior to starting the snatch test, perhaps with a bit of extra hold at the top position. This helps warm up the muscles for good snatches. Doesn't matter much if I do the areobic warm-up first or the snatch and hold first, but I've found that both help. Just keep it light -- don't fatigue yourself or take anything away from the performance of the test.
But keep in mind that no one will provide you a specific opportunity to warm up prior to the test at the cert, so you'll have to take it upon yourself to step away and do a few jumping jacks or whatever you decide is your ideal warm-up. Don't worry too much about the specific timing of it, but do what you can if you are able. Even a few bodyweight goblet squats or some "fast and loose" can help.