One "trick" I usually use with Lambdas - add a main() method that allows you to run the Lambda as a regular command line app. If you authenticate locally with a test environment in AWS, you can pretty much test everything in your code easily that way.

You can use this in your local dev env and even in integration tests. I don't need to do this a lot but sometimes it helps when troubleshooting.

Also, you might actually be building a tool which can be used from the command line. I recently took an existing command line tool (written in Go) and ported it to Lambda to expose it as a chatbot.