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

stable inclusion
from stable-v5.10.121
commit ec1378f2fa36f6e4a5042cca5ad6f415038dcda1
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5L6CQ

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ec1378f2fa36f6e4a5042cca5ad6f415038dcda1



--------------------------------

commit 213adc63 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>