testing subs and amp?

You could test at the car with the hood open, connect directly to the battery pos and neg.  Use the gauge wire that you would use to install the amp.  Use the enclosure that the subs will be in, too.  Jump a wire from power over to remote turn on to power up the amp (you wouldn't install like this but for a test you could do that).  You could probably even forego the inline fuse on the power wire if you're just using it for test purposes and will disconnect everything at the end of the day.

Otherwise, use any charged 12 volt battery out of the car, and keep the testing short and sweet.

This is the way I normally view a situation like that:  where you have equipment from an "UN" source - (UNusual, UNsubstantiated, UNauthorized,UNverified...)

• Equipment will be installed anyway, and for your total power needs there is the installation of power and ground wiring that has to be done.
• As you're doing now, the enclosure has to be built.
• Damping has to be done, anyway, for any system.

It's simplest to presume that everything works fine and go systematically through your checklist of installation procedure.  When you finally get to the finish line and reconnect that neg terminal on the battery so that you can "power up", that's when you will know if something is amiss.  Unless you have a test bench in your shop set up to easily do this at any time, the procedure is not worth the time invested, and you run further risk of damaging equipment because of the "rigged" nature of doing the testing.   So you just install the stuff, troubleshoot if something doesn't work....and replace the dud piece of equipment.  Good luck!

At my shop, we'll test the equipment customers bring in while they're present.  That way we won't have any issues with them claiming "it worked before."