I know these suggestions are all super-obvious, but, just in case you haven't thought of any of them:
1. I don't know about skirts and tights, but keeping other parts of you warm will help.
2. Heat the boots up thouroughly, inside and out, with a hair drier, before you put them on.
3. Replace the manufacturer's insoles with ones made from closed cell carpet foam, cut (in 3 dimensions) to match the appropriate shape. Make sure you skate on these at other sessions, to make sure they provide the right support and fit everywhere. Closed cell carpet foam is a very good thermal insulator, much better than leather and other typical insole materials. But! For some people it may be too warm.
4. This is probably too obvious to mention, but while you are waiting around, can you wear gloves, a hat, and perhaps a (long ?) coat, that you hand to your coach just before testing? (But not so warm as to make you sweat, as that is counter-productive.)
5. Many rinks have heaters or heating ducts in the air over or near the ice, that help keep down the relative humidity and fog. Can you stay near one?
6. If you sweat during warm-up, wipe it off with a small towel.
Eat lots and lots in the weeks before the test, so you have lots of body fat?