Tests with CgroupPath should choose a different one than '/cgrouptest'

[alban]

When looking at PR #631, I noticed it uses the following cgroup:

cgroups/cgroups.go:     AbsCgroupPath = "/cgrouptest"

and the spec says:

Runtimes MAY consider certain cgroupsPath values to be invalid

Runtimes might run under a systemd OS and in that case they should follow systemd guidelines to create the cgroup: CGROUP_DELEGATION.md

There is currently 3 supported ways using systemd:

• the integrated way: e.g. AbsCgroupPath = "/machine.slice/mycontainer.scope"
• the island way: I am not sure if that can be expressed with cgroupsPath?
• the continent way: e.g. AbsCgroupPath = "/ocitest.scope/mycontainer"

If systemd-nspawn implements the OCI runtime spec, it will consider the /cgrouptest cgroup to be invalid. The certification tests should not fail but use a cgroup path that is not considered invalid by the runtime.

/cc @poettering @iaguis

[zhouhao3]

If you have any good ideas, you can open a PR to solve this problem.