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
Add vtlock command for tests to ensure graphics exclusivity
Don't explicitly include core.
Don't override the log fd
Add xephyr and xorg tests.
Fix exit inside tests.
Tweak test timing; unmount-chroot is still an issue.
Fix xorg/xephyr test command
test/run.sh: Ignore backup copies of tests

Some editors create a backup copy of the file, ending with a tilde:
ignore those files.
test/35-xorg: Dump OpenGL+dri information