cpuacct/run_cpuacct_test.sh: fix the order of cleanup and task_kill

When there are running tasks in the child group of cgroup, the child
group can't be removed. So fix the order of cleanup and task_kill in
run_cpuacct_test.sh.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>