[DOC beta] Cleanup ember-testing documentation.

Helper Syntax Highlighting

Before:

![screenshot](https://www.monosnap.com/image/yPzfGQTdTiZXlRpNuduM2orTs.png)

After:

![screenshot](https://www.monosnap.com/image/lUF5fcv70K8sH92X3epLjpEaM.png)

registerAsyncHelper

Before:

![screenshot](https://www.monosnap.com/image/OICLOCKAoqvapb5FIecAg2QfP.png)

After:

![screenshot](https://www.monosnap.com/image/tebGA4uwb8wMhnbEzMg8hzks5.png)

registerHelper

Before:

![screenshot](https://www.monosnap.com/image/3GXahrtQ20EYs8wKbCWJw82ws.png)

After:

![screenshot](https://www.monosnap.com/image/K4sLcekKWKURKuFaExH5DYur2.png)
Merge pull request #4014 from rjackson/fixup_test_docs

[DOC beta] Cleanup ember-testing documentation.