cbuildbot: run new signer test script

We've got a new helper script for executing signer security tests.
Include it as part of the release build so we know when they fail
and so devs can test themselves.

BUG=chromium-os:19543
TEST=`cbuildbot lumpy-release` passed and ran signer tests
TEST=`./buildbot/run_tests` passes

Change-Id: Ic0b9a7dc7cef929310050cf0d6bd7ea7b4a0413a
Reviewed-on: https://gerrit.chromium.org/gerrit/34327
Reviewed-by: Ryan Cui <rcui@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 2b2404b..cf79264 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -384,6 +384,7 @@
     config = configs.get(board, self.build_config)
     stage_list = [[stages.VMTestStage, board, archive_stage],
                   [stages.ChromeTestStage, board, archive_stage],
+                  [stages.SignerTestStage, board, archive_stage],
                   [stages.UnitTestStage, board],
                   [stages.UploadPrebuiltsStage, board, archive_stage]]