targets/xorg: Downgrade mesa libraries on sid/jessie

This is trickier than precise, as libegl1-mesa depends on
libwayland0=0.85.0-2. We manually fetch and install the packages,
then hack the dpkg database to remove the dependency.
targets/xorg: Make sure invalid release ends in error.
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