selftests: tls: add helper for creating sock pairs

mainline inclusion
from mainline-v5.16-rc3
commit a125f91f
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56S5K?from=project-issue

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a125f91fe783a79c9d990fe6e28a3da8d9a0857f



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We have the same code 3 times, about to add a fourth copy.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>