skip prune-after-tasks-die test with premounted cgroups
skip prune-after-tasks-die test with premounted cgroups

Because our release-agent won't run in that case so cgroups will
in fact not be pruned.

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

hallyn/cgmanager-pkg-ubuntu,udeved/cgmanager,hallyn/cgmanager-pkg-ubuntu,lxc/cgmanager,hallyn/cgmanager,lxc/cgmanager,udeved/cgmanager,hallyn/cgmanager