Add a few more content-for placeholders.

* Add `{{content-for 'body'}}` to `app/index.html` and
  `tests/index.html`
* Add `{{content-for 'test-head'}}` to `tests/index.html`
* Add `{{content-for 'test-body'}}` to `tests/index.html`
Merge pull request #2236 from rwjblue/content-for-sprinkles

Add a few more content-for placeholders.