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".