Add support for triggering gold tests. This requires a specific service account and luci config. Also includes a few other minor improvements to trigger.py. Bug: angleproject:5966 Change-Id: I8f60b57cd7136fb4ffd4d6243fda9134ba5961fb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2933600 Reviewed-by: Brian Sheedy <bsheedy@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>