Unless you have special service PRVs, i.e. Oxygen Service, a clean room is not neccessary. If you do have O2 Service PRVs, you need special Test Equipemnt (Clean), probably SS, and clean test medium (Nitrogen).
Climate conditions vary. I have worked in Air Conditioned Shops and others which were not so much. No standard calls for such conditions. Offifial NBBI Certified PRV capacities for ASME PRVs are in SCFM, GPM or PPH which are corrected for Lab Conditions. Therefore, Test Room conditions do not necessarily need to meet any specific conditions re Temp and Atmospheric Pressure. That being said, high altitude, does affect set pressure to some degree.
JAC