targets/core: Set proxy variables based on previous proxy setting

On update, obtain *_proxy variables from the previously generated
environment, and export them after sanitizing the value (to make
sure we do not eval random harmful stuff).

Fixes #883.
Fix check for default variable values.
targets/core: Just source the new environment file from the installer

Simpler, should be safe enough.
Unit testing: add tool to import functions from scripts
Shuffle release order for each test.
xdg-utils is necessary for proper functioning of the croutonpowerd daemon
Merge pull request #911 from drinkcat/fix-proxy-env

targets/core: Set proxy variables based on previous proxy setting
Merge pull request #916 from dnschneid/randomrelorder

Shuffle release order for each test.
Merge pull request #915 from dnschneid/unittesting

Unit testing: add tool to import functions from scripts
targets/xorg: Downgrade some extra packages

mesa also provides some other packages (e.g. -dev and -dbg packages),
make sure we also downgrade the ones that break when we try to
install them.

See http://packages.ubuntu.com/source/precise/mesa for a full list,
a number of them probably work fine with more recent GL libraries
and may not need downgrading (e.g. libgbm), so we do not downgrade
them.
Merge pull request #922 from drinkcat/mesa-add-missing-pin

targets/xorg: Downgrade some extra packages
targets/xorg: lts-saucy will disappear soon, use lts-trusty
Merge pull request #925 from drinkcat/xorg-precise-lts-trusty

targets/xorg: lts-saucy will disappear soon, use lts-trusty
targets/audio: Fetch libsbc 1.1

libsbc 1.0 was provided by raring, which is not provided by mirrors
anymore.
install_mirror_package: When arch is set, install both native and arch

This is necessary as dpkg multiarch does not allow mismatching versions
on different architectures (e.g. you cannot have libsbc1-1.0:i386 and
libsbc1-1.1:amd64 installed at the same time).

Even --force-all in dpkg does not allow that.

We fix the problem by installing both packages at the same time in
install_mirror_package.
test/tests/30-audio: Test that install_mirror_package is able to update libsbc1.

Force installing a newer version, and see if it gets downgraded.
tests/30_audio: Improved install_mirror_package test
Merge pull request #933 from drinkcat/libsbc-fix-version

targets/audio: Fetch libsbc 1.1
tests/18-upgrade: Remove quantal and raring tests

They are gone. Forever. :-(
saucy unsupported, add utopic

Still disappointed they didn't choose undead unicorn as codename...