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: NWaiman Long <longman@redhat.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>

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