cli: create patterns for cross-account integration tests

The AWS CDK CLI supports a wide number of auth scenarios. The most common of the "non-standard" setups are cross-account deployments.

We need to come up with a repeatable pattern on how to define integration tests for cross-account deployments. This could be as simple as an example integ test and a few helper functions. Or it could be a "mini-framework". Or a service Either way, let's keep it very tightly focused on our needs.

See also aws/aws-cdk#31938