Fix for shadowed _ dep and broken AMD tests

The test AMD page isn't loading underscore correctly (passes `undefined`), and the underscore loader in the main backbone.js closure shadows the output from the UMD loader completely. Fixes these two bugs.

We can remove the `shim` config from the amd-test page with the next underscore release, but we should leave it for now as an example.

Edit: Also copied over element fixtures from test/index.html to replace the ones that were out of date.