autopkgtest: Run tests as root

mkinitramfs etc. are normally run as root, so it probably makes sense to run the tests as root. Additionally, on Debian armhf we now need to run as root in order to create dummy files under /lib/firmware.

Add "Restrictions: needs-root" to all tests.

Since modifying /lib/firmware is potentially breaking, also add "Restrictions: breaks-testbed".