au_test_harness: adapt to chromite CommandResult changes

Since
https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1818781
CommandResult.output is a compatibility wrapper for .stdout, and it does
not have a setter. Switch to .stdout, and stop trying to modify the
object unnecessarily.

BUG=none
TEST=none

Exempt-From-Owner-Approval: CQ is broken
Change-Id: I8c1d160ad6fc390ec838e790d44638f5b7416899
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/1830348
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Commit-Queue: Brian Norris <briannorris@chromium.org>
1 file changed