move to peer dependencies and fix test

[elthrasher]

Fixes https://github.com/boyney123/cdk-eventbridge-socket/issues/4

I noticed this project had pinned CDK dependencies to 1.120.0. When those are listed as direct dependencies, all consumers of the lib need to use that exact version. It's a bummer for CDK which will be fixed with the 2.0 version. For now, it's a better practice to list those dependencies as peerDependencies which lets the consumer install their own versions and warns them if they don't (npm 7 does this instead).

Also looks like some churn in the test snapshots. You might get more stability using the NodejsFunction construct.