Replace auto-load with "loader" function

[trentmwillis]

I think we should move away from having the tests "automagically" load; I'd like to see us move to something more explicit.

We should provide a "loader" function that consumers invoke from their test-helper.js file. This should allow us to remove the ready logic and the setTimeout while also providing an easy way for users to reason about how their tests get loaded. It would also make it easier to pass options to ember-cli-qunit without having to use the global QUnit.config object.

Thoughts?

Originally brought up here: #155 (comment)

[rwjblue]

Yes, agreed.

We should move https://github.com/ember-cli/ember-cli-qunit/blob/master/vendor/ember-cli-qunit/test-loader.js#L24-L68 to our own addon-test-support directory (and rewrite to use normal import statements), then have consumers do something like this in thier test-helper.js:

import { TestLoader } from 'ember-cli-qunit/test-support';

TestLoader.load();