Update Auto Test docs about supported buildpacks
Problem to solve
Specify what options users have to run automatic application tests with Auto DevOps using Cloud Native Buildpacks.
As a developer, in order to deploy with confidence, I want to run automated tests on every commit. As a developer, in order to focus on my core job, I want tests to be set up automatically.
Auto Test provides the solution to the above problems. As we're moving from Herokuish to Cloud Native Buildpacks, we would like to keep Auto Test functionality compatible with CNB buildpacks. Today, CNB does not have an RFC for this topic. Until an RFC is accepted, we want to document that Auto Test supports only the Heroku TestPack API.
User experience goal
User can find documentation around supported buildpacks.
Update the Auto Test docs to be very clear that Auto Test still uses Herokuish even if Auto Build uses Cloud Native Buildpacks, and that it expects a buildpack that implements Heroku's Testpack API. There is already such a note on Auto Build using Cloud Native Buildpacks, but the Auto Test section is a good place to add more details.