Voyager integration tests

[sdumetz]

To reliably keep adding features without breaking the growing userbase it becomes urgent to have some sort of integration testing that checks updates for regression before deployment

Puppeteer should be able to handle this.

While most tests should focus on functionality and should be easy to implement with the profusion of accessibility properties provided by Voyager, an image snapshot comparison tool will ultimately be necessary to check for proper render state. jest-image-snapshot is a popular candidate. The more lightweight chai-screenshot would help reuse previous work on puppeteer/mocha/chai integration tests from other projects. It uses pixelmatch under the hood, which seems OK for our case.