Simplify runtime dependencies.

* Since we are now distributing proper ES6 builds, we no longer need to use Handlebars and broccoli-funnel as dependencies (they are only needed for building and not by our consumers).
* Moved `test/support/assertions.js` to its own package `htmlbars-test-helpers`.  This allows **much** easier sharing with Ember (and can remove [this special casing](https://github.com/emberjs/ember.js/blob/f464514b0a8f6e8ebdf38d9e69014f94d9047e51/Brocfile.js#L656-L667)).

For use in Ember here: https://github.com/emberjs/ember.js/pull/9592.