I've resealed many tanks over the years, though I'm no expert. My gut instinct is that the silicon will take a lot longer to cure at such a low temp, a week may be good, but it might need even more time.
I wouldn't risk doing your water test outside in sub freezing weather. Things can happen, as others have noted. When I was a kid in the 1980's I watched as my friend tried to hose out his 29 gallon tank on a very cold New York day. The hose water hit the ice cold tank, and the tank immediately exploded into a thousand little pieces.
I would do your water test inside. I'd put the tank on its stand making sure everything is level. Unlevel stands/tanks can lead to tank breakage. Then, just go very slowly with the water. Fill it just a couple of inches and wait and see. No leaks? Then add another couple inches of water and wait and see. You don't have to fill the whole tank up all at once, and if you do have a leak it will likely not be completely catastrophic, but likely just a slow leak from a small part of the seam.
Also, before you applied your silicon, did you clean the glass with acetone or alcohol? Using a solvent like these before the silicon really helps to get a good seal.
Good luck!
