Yes, you can test code found or developed before the jam. There would be no real advantage if you could not test code while it is developed, because you might just have to scrap it all and start over anyways.
When I test for my games, I always put them in a separated testing scene specifically dedicated to the function, like "Movement" or "LevelDesign". Just don't create a scene like "Level" and start putting everything into that scene, because at that point you are "making your game".
Platformers are rarely made from scratch (no pun), I am using a character controller from GitHub for example (I can provide a link if you would like). The point of this jam is to take assets and see how you can assimilate them into a finished (or mostly finished) game, with your touch to make it unique.
