This patch introduces a dashboard that records test results and
uploads them to https://github.com/aslushnikov/puppeteer-flakiness-dashboard
Since many bots might push results in parallel, each bot pushes
results to its own git branch.
FlakinessDashboard also generates a simple README.md with a flakiness
summary. If this proves to be not enough, we can build a website that
fetches flakiness data and renders it nicely.
