Create test helpers for downstream projects to use

[ghost]

Feature request

Add a set of test helpers to resolution that projects like Pipelines can use to easily stub / mock ResourceRequest objects, Requester and ResolvedResource implementations, and errors.

Use case

Currently Pipelines maintains its own set of test helpers for stubbing resolution types and also code for mocking the base64 encoded response to a ResourceRequest object: https://github.com/sbwsg/pipeline/blob/pipelineref-remote-res/pkg%2Freconciler%2Fpipelinerun%2Fpipelinerun_test.go#L8770-L8791

It would be great if Pipelines did not have to host this code and instead this kind of test support was available from resolution test helpers. Once these helpers are written Pipelines should be updated to use them.

[abayer]

Oops, fixed this in #51 but forgot to call it out.