[testing] Implement an end-to-end test suite for Counterfactual

[joelalejandro]

We'll add a new package called greenboard. Its goal is to test a series of interaction flows simulating user input for the scenarios described in #1852.

Task list

Initial work was done in the joel/e2e branch (see #1906).

• [x] Create package greenboard
• [ ] Install and configure selenium for CI runs
• [x] Prepare a script that allows to pack an updated version of the Metamask Extension with the CF vendors to install it in Chrome
• [x] Prepare a script that bootstraps a Metamask wallet
• [ ] Write tests for the scenarios (see #2134, #2135, #2136)
• [x] Add documentation on how to use and extend Greenboard (see #2101).

[joelalejandro]

We had to scrap the idea of testing with Cypress because they're not ready for Chrome Extensions yet (see cypress-io/cypress#1965). So we're moving forward with Puppeteer instead.

[joelalejandro]

Once again, scrapping an idea :man_facepalming: Puppeteer offers no API to interact with the browser toolbar, so we wouldn't be able to sign messages or deposits. Moving on to Selenium.