tests: cgroup02 should use --cgroup-root on dump

To mimic the environment that systemd is in when it causes the problem, we
preserve a prefix cgroup path across checkpoint and restore which the tasks
live below, and then we open a fd to this path. Without
--cgroup-root /prefix on dump, this test fails:

(00.030429)     32: Error (criu/files-reg.c:1487): File home/ubuntu/criu/test/zdtm/static/cgroup02.test/zdtmtst/prefix has bad mode 040600 (expect 040777)

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>