Refactor test helpers

[cibernox]

There is several ways in which the test helpers should be improved:

1. The idiomatic place to put test helpers is inside /addon-test-support. We should move them. To ease the transition, we should import and reexport with a deprecation from /test-support for a while.
2. Afaik Ember.Test.registerHelper is not required (I think it doesn't even work!) with the new test syntax proposed in Rethink Acceptance Testing emberjs/rfcs#268 and shipped already in ember-qunit 4.2.0. Helpers. Those helpers should be explicitly importable with import { expectTranslation } from 'ember-i18n/test-support/helpers';. That means they would also work in integration tests too.
3. Make the helpers not rely on jQuery, as some apps don't use it anymore, and the goal is to not include it by default in future versions of Ember.
4. IMHO, I believe a custom assertion like assert.hasTranslation('#selector', 'login.button.submit', { count: 4 }, 'Optional assertion description') would be more idiomatic. Although personally I would use a mix of qunit-dom and the t test helper: assert.dom('#selector').hasText(t('login.button.submit', { count: 4 }), 'Optional assertion description')

This radar is so I don't forger, but I'll try to work on those points myself over Xmas. But if someone wants to beat me to it, just leave a comment here and do it.