cli: create patterns to test interactive commands

[mrgrain]

The AWS CDK CLI is a command-line tool supporting interactive and unattended operation. However due to complexity of testing interactive input, all our tests (unit & integ) are currently only testing unattended operation. In some cases this is good enough, however in other cases the code paths are significantly different enough to warrant explicit testing.

We need to come up with a repeatable pattern on how to define integration tests for interactive input. This could be as simple as an example integ test and a few helper functions. Or it could be a "mini-framework" to mimic the API of browser testing frameworks like playwright. Either way, let's keep it very tightly focused on our needs.