Add full integration test using `vercel dev` or testing the actual preview deployment

[gr2m]

I upgraded Probot to v12 via #381 which actually broke the app, without any tests failing. The problem is that we only testing the app by directly importing app.js. We should add at least one full integration test where we either start the server to with vercel dev and send a POST /api/github/webhooks request to the local server, or send the same request to the Vercel preview deployment, once it's ready. The former is easier, the latter is better.

[tenshiAMD]

@gr2m Can I give this a try?

[gr2m]

All yours 👍🏼