[Dev Happiness] Add a simple set of testing functions to mock webhooks

On a separate project I work on, I have set up a small script that allows you to specify a webhook mock, and a function, and it calls that particular function with the mocked hook.

For instance, I have a few JSON files containing `push` webhook request JSON details.

On the CLI, I can call `./tools/testhook ` and it will call the correct function and pass in the webhook data.

I'll be honest, I haven't read through the code yet, but that should make debugging a *lot* easier.

If people are keen for this, I'll open a PR.