Require host to provide testing fixture DOM elements.

[rwjblue]

Instead of ember-qunit automatically injecting some DOM elements into test-body (via the contentFor hook) this requires the project itself to add these DOM elements.

This dove tails with the changes to make qunit a peer dependency as this now puts the app in full control of the testing fixtures.

See:

• Remove manual DOM cleanup code after each test. ember-test-helpers#911
• Move #ember-testing-container into #qunit-fixture element. ember-test-helpers#910
• Require project to provide @ember/test-helpers and qunit. #738

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For applications migrating, add the following just under your existing {{content-for "test-body"}} entry in your tests/index.html:

    <div id="qunit"></div>
    <div id="qunit-fixture">
      <div id="ember-testing-container">
        <div id="ember-testing"></div>
      </div>
    </div>