[autotest] Let log socket server use an unused port

BUG=chromium:383370
TEST=unittest

Change-Id: I56df36cb230d4f4f81f8ce1525b417a132c15aaf
Reviewed-on: https://chromium-review.googlesource.com/203542
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Jiaxi Luo <jiaxiluo@chromium.org>
Reviewed-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
5 files changed