Add support for testing as a nested addon

[kellyselden]

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This is a long shot, as it probably won't be an easy task, but...

Nested addons (addons inside addons) have a number of gotchas. If we could insert the current addon into a dummy addon, which in turn is inserted into a dummy app, that would be epic.

[trentmwillis]

For ember-engines, nested addons were a very common test case so we built some helper abstractions to do this. We should likely find a generic way to pull them into this addon.

See here for instance.

[kellyselden]

@trentmwillis Yes! That code chunk looks like the right path.