tests/cgroup - Add more helpers for cgroup delegation

tests/cgroup - Add more helpers for cgroup delegation

Add more helpers to work with systemd cgroup delegation, this patch
adds:
- create_and_validate() to create cgroups and validate if they were
  created under the delegate systemd subtree.
- exists() is called by create_and_validate() to perform the
  validation part.

Suggested-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>