Generate test scaffold?

If I add test generation support, I will probably do it as a sub-generator.

I see two test scenarios:
- Full blown test with AVA. Helpful if you plan to ship the codemod.
- Simple tests using just `assert` module and logging statements (no runner). Stays true to the "install no additional dependencies" goal, and is useful for quickly testing/debugging a throwaway codemod.

No files, tests, or entry points are named. Start by locating the generator's existing sub-generator structure, then clarify whether the scaffold should support AVA, dependency-free assert and logging, or both. Done requires an agreed generated test shape and a defined way to verify each supported scenario.

Written by the indexing model from the issue text.