I've seen some revolvers that only have 3 chambers dirty from test firing, and I've also seen some that have every chamber dirty. I worked in a gun retailer and opened up hundreds if not thousands of boxes of guns. Every one of them is fired from the factory and they're never cleaned afterwards.
Colt revolvers and autos have the Verified Proof stamp which is a tiny "VP" in a triangle on the left side of the trigger guard.
Revolvers have it on the left-front of the guard, autos usually have it on the left-rear.
This signifies that the gun did pass a proof test, so yes, they were all fired at the factory.
I used to frequent a local gun shop, now defunct, that sold a considerable number of new Colts. (Obviously before Colt almost dropped all revolver production) Once in a while the shop would get a revolver that was really dirty with every chamber having been shot more than once. The shop employees termed these filthy specimens "Lunchtime Guns" meaning instead of just proof firing, someone had some free period fun with the gun. Why not? I would have if employed by Colt back then.
I just handled a new Cobra that came straight from the distributer. It was fired at least 6 and probably 12 shots and was not cleaned afterwards. Colt may reject such a gun and send it back from test fire for an adjustment or rework and then re-test fire it. So there could be a lot of shots fired from any particular gun. I have seen video of Colt test firing Gov't Models and then a quick clean of the barrel on a fixed brush arrangement.