Auto Test needs an alternative to `herokuish buildpack test`

Problem to Solve

In order to move off Herokuish (which is essentially deprecated), we need an alternative solution for Auto Test. Auto Test currently leverages

herokuish buildpack test

Proposed solution

Implement the equivalent by leveraging the work done by Cloud Native Buildpacks (CNBs), which we envision superseding herokuish buildpack test.

Unfortunately, test suite detection is not yet part of the CNB spec, nor is there an RFC. However, the CNB team has expressed interested in such developer oriented features (some even used to be in the spec).