selftests: cgroup: Run test_core under interfering stress

test_core tests various cgroup creation/removal and task migration
paths. Run the tests repeatedly with interfering noise (for lockdep
checks). Currently, forking noise and subsystem enabled/disabled
switching are the implemented noises.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>