feat(tests): add test-nightly.sh script for CI

To be used by our nightly buildpack / Dockerfile regression testing
jobs at http://ci.deis.io/view/example-apps/.

This is what is being used at http://ci.deis.io/view/example-apps/job/test-integration-go/ currently. Seems to work, so I put it in a script under version control.

[skip ci]
No, I meant to raise an error there. These should be supplied externally to the script so we don't make too many assumptions, and so the same script can be used for local testing as well as in the CI system.