We can test webhook callbacks with a little http server and a localtunnel.
Run server.js to spin up a local http server on port 8080. It just exposes a top-level route and will print the received header and payload of any request. Then create a localtunnel to have a non-local url that proxies all requests to localhost:8080.
First, install dependencies with npm install.
Then start the express server with npm start.