tests: add a test for memory/devices "specialness"

v2: add cgroup04.hook which removes the cgroups in between, so things are
    actually restored :)

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>