Provide a path for integration tests

[dmikusa]

Split from #53.

We should consider adding utilities for integration testing buildpacks (possibly in a separate library) that possibly uses pack and docker. Example of something like this https://github.com/paketo-buildpacks/occam

Alternatively, we could endorse using occam and update docs to reflect this.

[loewenstein]

Alternatively, we could endorse using occam and update docs to reflect this.

We really should not reinvent occam. Either use it from where it currently lives or approach the occam maintainers if they could see moving into the cnb project.