Switch to treeForAddonTestSupport

[rwjblue]

This is what we do in ember-native-dom-helpers:

  treeForAddonTestSupport(tree) {
    // intentionally not calling _super here
    // so that can have our `import`'s be
    // import { click, fillIn } from 'ember-native-dom-helpers';

    const Funnel = require('broccoli-funnel');

    let namespacedTree = new Funnel(tree, {
      srcDir: '/',
      destDir: `/${this.moduleName()}`,
      annotation: `Addon#treeForTestSupport (${this.name})`,
    });

    return this.preprocessJs(namespacedTree, '/', this.name, {
      registry: this.registry,
    });
  }

The benefit here is that this will now put the code in test-support.js instead of vendor.js. This allows building prod for deployment including tests (for testing against a live site for example).