targets/xorg: Install lts-saucy drivers on precise for Haswell support

We first pull in `lts-saucy` replacement drivers, then `xorg`:
- Doing it in that order still pulls in `xorg` dependencies like
  `xterm` and `xinit`, and avoids the need to specify them manually.
- Removing ``--minimal`` from the `lts-saucy` line also works, but
  also pulls in stuff like an updated kernel, which we don't want.
- Installing `xorg` after `lts-saucy` does not install old drivers
  (so there is no useless download).
- On existing chroots, installing `lts-saucy` uninstalls old drivers,
  and marks a number of packages as not required, like `xterm` and
  `xinit` (which would be cleaned later by autoclean). Reinstalling
  `xorg` does not install new packages, but makes sure these packages
  are not autocleaned.
targets/xorg: Install lts-saucy xorg whenever kernel != 3.4
Merge pull request #890 from drinkcat/precise-haswell-xorg-lts-saucy

targets/xorg: Install lts-saucy drivers on precise for Haswell/Baytrail support
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
installer/ubuntu/prepare: detect_mirror function

Detects the current mirror.
targets/xorg: >=saucy, kernel 3.4, pin down precise mesa drivers.

On saucy onwards, if kernel version is 3.4, manually pin down old mesa
libraries from precise, as new ones require version >=3.6.

Fixes #704.