br: use random port to avoid flaky test (#65186)#70012

ti-chi-bot wants to merge 2 commits into

pingcap:release-8.5pingcap/tidb:release-8.5from

ti-chi-bot:cherry-pick-65186-to-release-8.5ti-chi-bot/tidb:cherry-pick-65186-to-release-8.5Copy head branch name to clipboard