[BUGFIX release] Enable testing of production builds.

Update the build process to emit `ember-testing.js` which contains the
`ember-debug` and `ember-testing` packages.  This file can be include
when running tests against production builds of Ember (`ember.prod.js`).
Merge pull request #9411 from rwjblue/emit-ember-testing

[BUGFIX release] Enable testing of production builds.