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]
do we need to set this envvar if it isn't set?