cgroup/cgroup_regression_test.sh: fix an issue about removing directories

After running test_$cur cases successfully, executing
"rmdir -p cgroup/*" outputs "rmdir: failed to
remove cgroup/*: No such file or directory" error
message, this is because cgroup is an empty directory
here. Fix it.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>