In the documentation for in app billing (v3), it says: \"Upon a successful purchase, the user’s purchase data is cached locally by Google Play’s In-app Billing service.\"
Couldn't you create a new emulated android install from the android development kit... then save that as a "snapshot" to before you logged in and it created all of the billing cache? Then you can just fire up a fresh instance of that VM and run your code without there already being a billing cache.
EDIT: If you cannot do so with the emulator, I'd suggest having a phone that is rooted and rom'ed. Take a full system snapshot with Rom Manager (Nandroid Backup) and then test out your app. When you wish to reset, restore your backup via Rom Manager and you are back to before you logged into the market (and activated the billing cache). It honestly takes longer for your phone to reboot than to actually restore the data... so it's pretty fast (I do it all the time since I'm an avid root/rom-er.