On the German sister forum
e-reader-forum.de there is a Pocketbook employee actively participating (user name
Forkosigan). If you send him a message he adds you to the Beta distribution list.
I wouldn't call it a testing program, though. Yes, you get the beta version and of course you can report bugs, but there isn't any feedback on what has been changed in beta versions, which bugs are being addressed or what you should look out for.