kseltest/cgroup: Make test_stress.sh work if run interactively

Commit 54de76c0 ("kselftest/cgroup: fix test_stress.sh to use OUTPUT
dir") changes the test_core command path from . to $OUTPUT. However,
variable OUTPUT may not be defined if the command is run interactively.
Fix that by using ${OUTPUT:-.} to cover both cases.
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/cgroup/test_stress.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
-./with_stress.sh -s subsys -s fork ${OUTPUT}/test_core
+./with_stress.sh -s subsys -s fork ${OUTPUT:-.}/test_core