I installed testdrive 1.21-0ubuntu1~ppa3 on a machine with non-free virtualbox-3.1. Since I didn't have any of the VM programs in the dependency list [qemu-kvm | kvm (>= 1:84+dfsg-0ubuntu12.4) | virtualbox-ose (>= 3.0.8)], APT installed kvm and all it's required depencencies. In ~/.testdriverc I added 'VIRT = "virtualbox"', and tried to installed Ubuntu Desktop via testdrive. When the new virtual machine tried to boot, I got an error from VirtualBox about a corrupted DKMS module as a result of the kvm installation (sorry for not having the exact error message). Additionally, none of my other virtual machines would boot. Luckily, after uninstalling testdrive + dependecies, and reinstalling virtualbox-3.1, my VMs started working again.
Looking at the testdrive script it seems like the program will work with virtualbox-ose 3.0 & 3.1, so it will probably work with the non-free version. Could 'virtualbox-3.0' and 'virtualbox-3.1' be added as dependencies to testdrive so that it won't try to install other VM software? Those are the exact package names from 'apt-cache search virtualbox'.
This is my first time submitting a patch, so please pardon if I've done something wrong.
The attached bzr patch to 'debian/control' should add virtualbox-3.0 and/or virtualbox-3.1 as dependencies. I don't know how to bundle the checked-out files into a deb file so I wasn't able to test thoroughly. However, I applied the same change to a downloaded testdrive deb file that I unpacked, modified, then repacked, and it installed without problems.