[Autotest] Copy only new data in /var/log folder after each test.

Copy all content in /var/log folder causes overhead in both test time and
test result storage. The /var/log folder's size can be over 40MB after a
test is finished. Code change is made to implement the diff logic to only
copy new data in /var/log folder before and after each test.

TEST=manually run autoserv with sleep test and verify new data in /var/log
folder is copied over to [test]/sysinfo/var/log_diff. Also use trybot build
trybot-lumpy-paladin/R28-3915.0.0-b741 to confirm autoupdate job's /var/log
is collected as expected.
DEPLOY=manual, before push to prod, this CL must be backport to all branches
being tested.

Change-Id: Ic78c5aa6c35eeab63bb3a0d8cb7095b7f55f217b
Reviewed-on: https://gerrit.chromium.org/gerrit/47138
Reviewed-by: Scott Zawalski <scottz@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
3 files changed