buildserver: support installing packages from Debian/testing
Sometimes, a build process requires newer versions of build tools than are available in Debian/stable. Oftentimes, using the package straight from Debian/testing works fine when a package is not available as a backport. libtool 2.4.6 is needed for building VLC, so it is one example of this.
The preferences file sets up the apt "pinning" so that all updates are not installed from testing, only the packages that are requested by adding "/testing" after then package name.
closes #224 (closed)