chromite: Add ImageTestStage to run tests on built image.

This stage is run in parallel to other stages after BuildImageStage. It
will launch test_image outside the chroot. This stage is forgiving.

Tests live in chromite/cros/tests/image_test.py.

BUG=chromium:382708
BUG=chromium:385355
TEST=unittest
TEST=test_image path/to/image_dir
TEST=test_image path/to/chromium_image.bin
TEST=cbuildbot --local amd64-generic-asan --nobootstrap --noreexec \
       --nouprev --nobuild --noclean --notests
     Make sure that ImageTestStage is launched.

Change-Id: Ia9f1123f9c533ad70a0091ee943d21cddc577ef0
Reviewed-on: https://chromium-review.googlesource.com/203698
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Nam Nguyen <namnguyen@chromium.org>
Tested-by: Nam Nguyen <namnguyen@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index ab15fd4..1ed20a2 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -538,6 +538,7 @@
         [release_stages.SignerTestStage, board, archive_stage],
         [generic_stages.RetryStage, 1,
          release_stages.PaygenStage, board, archive_stage],
+        [test_stages.ImageTestStage, board],
         [test_stages.UnitTestStage, board],
         [artifact_stages.UploadPrebuiltsStage, board],
         [artifact_stages.DevInstallerPrebuiltsStage, board],