[autotest] Save preserved log during autoupdate.

In following test, add a logdir to job.sysinfo.test_loggables to save files in
/mnt/stateful_partition/unencrypted/preserve/log

server/control/segments/provision
server/site_tests/autoupdate
server/site_tests/autoupdate_EndToEndTest
server/site_tests/provision_AutoUpdate

The logdir will enable autotest to collect the preserved logs and save to
results folder:
[test]/sysinfo/mnt/stateful_partition/unencrypted/preserve/log

BUG=chromium:276919
TEST=test_that 172.22.75.106 dummy_Pass -i lumpy-release/R34-5233.0.0
trybot build trybot-lumpy-release/R34-5238.0.0-b1752

Change-Id: Ia4beee8a43a3b9c0959ce00b514eda5f29f27f71
Reviewed-on: https://chromium-review.googlesource.com/182948
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
8 files changed