AWS Lambda is a very small interface. I'm only getting started, but I'm planning to keep all my application code in a separate project with no Lambda dependencies, test that as I would any application, and then have a thin wrapper that just wraps each endpoint in the Lambda interface.

This would also maintain as loose as possible coupling to the Lambda ecosystem, making porting easy -- I'm in Java-land, so that probably means Dropwizard. I'm not there yet, but I'm planning to maintain a parallel Dropwizard wrapper, to make local dev easy.