[autotest] Create a suite for testing push to prod

Add suite push_to_prod for cbf run.
Add a tool, site_utils/test_push.py, used to test if code is ready to push.

BUG=chromium:263928,285453
TEST=unit test
Test with a trybot build: trybot-stumpy-release/R35-5526.0.0-b373
AU part is verified with latest stumpy canary build.

Change-Id: Ia221f0a1aa0f1687daab5d8a20307435baa6bdfc
Reviewed-on: https://chromium-review.googlesource.com/66460
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
16 files changed