au_test_harness: clean up old constants reference

Use chromite.lib.constants directly since we've deleted
chromite.cbuildbot.constants.

BUG=chromium:997354, chromium:1072917
TEST=CQ passes

Change-Id: Ie36f57085bd0eb29d821b1dbb956f8dfc6ec7a0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/2158506
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
1 file changed