Support, addons, in-repo addons, in-repo engines etc

[RobbieTheWagner]

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This works wonderfully for testing with an app, but should we support addons, in-repo addons and engines, etc?

There are some helpers for in-repo addons and engines here https://github.com/ember-engines/ember-engines/blob/master/node-tests/helpers/in-repo-addon.js and https://github.com/ember-engines/ember-engines/blob/master/node-tests/helpers/in-repo-engine.js