cg: Don't close cgyard early when inherit cgroups

Fix for closed cgyard descriptor for changed cgroup was 9752c11d.
One more place left.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
zdtm: Sanitize cgroup00 test

Prepare it for multiple tasks by moving cg creation and
checking code into helpers.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
zdtm: Make cgroup00 test check several tasks and sub-cgroup

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>