A test setup for Lighthouse tests of Netlify PR previews.

Some notes:

• It tests Mobile and Desktop by doing two runs with additive: true and the preset appended to every URL (see GoogleChrome/lighthouse-ci#138). This is not pretty, but I think it's the only current way.
• It reports the summary result as a PR comment (2 markdown tables).

Note to self: This setup does not work for PRs from private forks. See https://github.com/bep/netlify-cypress-test for a workaround.