Monorepo Test Harness

[buschtoens]

#12 introduced some really naughty h4xx that patch the consuming host's ember-cli-babel.

We should add an extensive monorepo test harness to assert that this works reliably with:

• apps
  • without Embroider
  • with Embroider
  • with old dependency versions
• addons (standalone dummy app & when consumed)
• engines (standalone dummy app & when consumed)
  • bundled
  • lazily loaded

This is akin to the setup in ember-auto-import.