To properly test water rough in shouldn't all the air be bleed out of the pipes and then pressurize to 100psi to test. I had one plumber tell me the air in the pipe does not matter. Seems like it would be hard to find a leak if it does not blow the pipe apart.
I don't think I've ever pressure tested lines with anything but air...when give a choice. Use air, and if you lose pressure then look for the hissing. If you can't find the hisses, you can always use the Oatley leak detector on every connection, I've used diluted soap with good results.
when it comes to breakfast, the chicken is dedicated, the pig is committed.
I really don't think air in the lines matters because the air will be pressurized to the same pressure as the water pressure in the pipes. The problem is getting enough pressure to get a good test.
If you can get one, a hydrostatic pump gives the best test. This may sound like a big deal but it's really not. This is just a hand operated pump that has a water reservoir attached. You use a washing machine hose to pressurize the system through the washing machine box or a hose faucet. This allows you to pump the lines up to 200- 300 psi. If you have a leak, you WILL find it.
You can probably rent one of these for a few bucks if you call around.
I have tested with air a lot but if there is a lot of noise it is sometimes hard to hear a leak. If you have a leak while testing with water, you can see it. Also, when using plastic piping, especially CPVC, there is a danger of "shrapnel" if someone accidentally breaks the pipe while it is pressurized.