Bug 865287 - [sms] [unit tests] sms tests should use the real html from index.html - r=julienw

Adds a load_body_html_helper.js to our test suite which can replace the current document.body.innerHTML with the contents of the <body> from a loaded html file.  It will remove any <script> tags before doing so.  This is a **synchronus** operation:

example:
```
loadBodyHTML('/index.html');
// document.body.innerHTML has be replaced!