With pinggy.io you can test your Slack Webhook from localhost with just a single command. Replace the port 8080 with the port where your local development server is running and make sure it is ready to receive requests (e.g. http://localhost:8080/webhook-url).
Pinggy will give you a URL like https://ranlkfsbjkxry.a.pinggy.online. Use this URL to access the webhook.