It is very unlikely that you can downgrade without jailbreaking your device.
The only solution is to get the software images for the specific ios versions and run them in the Xcode Simulator.
Unless you are doing very hardware specific software ( i am talking driver level code) there is no need to worry about device testing on all iOS platforms. If it runs on one then it will run on others. Just downgrade the Xcode simulator.
Also 91% (Apple data source) people have upgraded to iOS 6 so you shouldn't worry so much about the rest 9% unless your app is like Whatsapp or going to be as famous as them.