I’ve identified one way to do this that would allow me to avoid breaking into those pipes. I could use inflatable test plugs with extension hoses with the whole thing shoved 4ft down through my new drain stub-outs. But the water would be on the “wrong” side of the plug, so I would need to inflate the plugs and then close the hose up inside the pipe with the water and then put a cap on my stub-outs. After the test, I’d have to create a vacuum in the vent pipe with another plug so I don’t have water gushing all over the floor while I remove the caps on the stub-outs to deflate the plugs in the drain lines.
What is the possibility of using a mission coupling say 11/2 or 2″ by 1″ copper or IPS?
When required to put a pneumatic / hydrostatic test (5 PSI or 10 ft head) I also use a balloon type of plug BUT I slip the plug past the pipe opening and use the mission coupling with a nipple and valve.
This way after the test rather then try to vacuum out the remaining water I open the valve slightly using a garden hose adapter and then can dump the water in a place where it wont cause any problems.
Besides a lot of times a decent inspector will ask you to drop the pressure in front of him to make sure you did have the pressure in the system as a lot of *********** cheat with defective gauges or non calibrated ones