kselftest/cgroup: fix test_stress.sh to use OUTPUT dir

Running cgroup kselftest with O= fails to run the with_stress test due
to hardcoded ./test_core. Find test_core binary using the OUTPUT directory.

Fixes: 1a99fcc0 ("selftests: cgroup: Run test_core under interfering stress")
Signed-off-by: Phil Auld <pauld@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>