buildserver: support installing packages from Debian/testing

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