Testing a tank with compressed air or any gas for that is not only a bad idea, even when the tank is filled with water first, then pressurising it with air. It is actually illegal, or it should be, certainly is where I live. Any gas including air will dissolve in the water, and when the tank lets go you still have the rapid release of stored energy. Small home or shop air compressors almost always suffer from internal rust. Mostly because we, the owners don't drain them often enough. In the humid climate that is so often found in the uS moisture condensation in air tanks is always going to be present. the tanks really need to be set up with a permanent drain. You van buy or even make an automatic drain valve.
For simplicity's sake, as I don't run my compressor very often I have set it up with a small drain valve that is permanently cracked open, just a small but continuous blowdown, and I live in a very dry climate, but I still get some moisture out of it. I'm hoping that will extend the life of my tank, I do intend to set up an automatic drain valve,but not sure when this will happen.