Core Reconciler tests and easy-to-reuse Resolver test harnesses

[ghost]

Feature request

Testing reconcilers requires quite a bit of setup code which we don't have yet for this project. This is an important short-term problem for three different classes of test:

1. The core ResolutionRequest reconciler, though relatively small, needs thorough test coverage
2. The "resolver framework" needs to have the contracts (interfaces) it exposes tested to ensure they work consistently for all resolvers.
3. Resolver developers need a pain-free way of spinning up a test harness so that they can quickly add coverage to new resolvers.

Use case

Primarily concerned here with supporting developers of new resolvers by giving them a pain-free-as-possible way for them to exercise their resolver code quickly and easily.

Since a resolver is just a reconciler under the hood hopefully we can find a way to provide a Resolver Test Harness that is general purpose for anyone's resolver development.

Inspiration

The Tekton Pipelines unit test suite includes a plethora of setup code that performs all the necessary injection gubbins to support testing their Reconcilers. This might be a good first place to look for inspiration: https://github.com/tektoncd/pipeline/blob/main/test/controller.go

[abayer]

/assign