[autotest] Take a screenshot when a test fails.

If X is not running, no screenshot will be taken.

BUG=chromium:383605
TEST=test_that 172.22.30.146 dummy_Fail
confirm screenshot is created, e.g.,
/tmp/test_that_results_rQiKvU/results-1-dummy_Fail/dummy_Fail.Fail/debug/dummy_Fail.Fail-fail-0.png

Change-Id: Ic044f611ab6f5d65d8ca973f9847346b3599891c
Reviewed-on: https://chromium-review.googlesource.com/204689
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
2 files changed