I'm not sure how the testmode does it's thing, but it may have done more than just erase something and make it not boot again. It should still boot to testmode again. Most devices like this should always look for a bootloader on the SD and go from there. The only way would be if you had an SD card with the right data to tell the unit to boot and copy to the onboard flash from scratch including reformatting it. But if that loader is horked, I think youre stuck.
Not sure if this would work, but since Pioneer sells SD cards specifically for upgrading with the 2.0 files on it, but you shou