tests: add vruntime to scheduling_group_demo
tests: add vruntime to scheduling_group_demo

During the first test, we expect vruntimes to be equal, and
printing them out makes it easy to verify this.
Message-Id: <8dd57992d0f5ea0d616ba2a446b9ba2810fc2ee9@scylladb.com>

dreamsxin/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar