Pressure tests are generally not conducted with any gas!
It's much too compressible, and the stored energy is high.
Pressure tests are usually done with water or another fluid with low compressibility.
Much less stored energy.
So if it fails, it fails with a phut, not a bang.
My father is the proud owner of a 1915 steamroller. Every spring he has his annual appointment with his boiler inspector. This starts with a visual inspection, using cameras and the mk1 eyeball, then progresses to a hydraulic test, where water is pumped into the boiler to a greater pressure than the normal maximum working pressure - usually about 20% over, I think. (I could be wrong with the 20%, it may be 25% or more!)
If there are no leaks with water, the inspector makes an appointment for another visit to test it under steam. They don't do that the same day, as it takes a while to get the water level back down to normal and to raise steam to working pressure - 125 p.s.i. in the case of the father's engine. Once the boiler passes the steam test, the inspector issues a certificate stating that the boiler is passed safe for 14 months.
Everybody who has a steam powered vehicle has to pass these tests, the insurance companies don't mess about when it comes to insuring something that can cause a *lot* of damage when it goes **phut** - it makes letting the magic smoke out look like a Sunday school picnic, if you'll pardon the mixed metaphor!