There are three vitrified test chambers in 1950's section. Those are located in 'sp_a3_03' (the level with toxic goo lake, lobby and Pump Station Alpha) right under a white sloped surface, which you use to fling yourself to Pump Station Alpha. All you have to do, is to press E on small panels which play audio, next to the vitrified test chamber doors. Those are very easy to find, because they are easily visible and have a white sloped surface which you need to use.

There are last three vitrified doors in a secret hallway in the offices located higher than the control room in 'sp_a3_transition01' (the level in which you find and attach PotatOS to your portal gun). The automated door to the hallway is hidden by furniture. After crossing the door, you are in hallway in which two test chambers are on the left, and one is in front of you. Just press E on audio panels and you have the achievement!

Thanks for reading my guide ;3
I hope it helped you get the "Door Prize" Achievement