Remove need for app.json to define test environment

[douglascayers]

Currently, developers add the app.json file to their project to hook into Heroku Flow. However, the test scripts property reveals scripts and knowledge specific to the buildpack itself:

./vendor/sfdx/release.sh ci-$HEROKU_TEST_RUN_COMMIT_VERSION

This feature request is for the buildpack to provide a default test-compile and test scripts (as mentioned here) that (a) can be called by both Heroku CI and #33 and (b) allow the removal of test scripts from app.json unless a developer needs to tweak the behavior.

[wadewegner]

Similar to #35, it'd be nice to hear from @ike-delorenzo and @devarispbrown if there's a better way to approach.