Simplifies testem configuration to reduce duplication

Instead of specifying the source and serve files, this change
utilizes the test_page option to test index.html directly and load
all the scripts as specified in that file.

  - Removes src and serve options from grunt testem task and from
    testem.json
  - Adds testem.js to index.html as per
    [testem docs](https://github.com/airportyh/testem#include-snippet)
  - Adds routes to testem configuration so that testem can correctly
    locate test files and assets as referenced in index.html

cc @cavneb @stefanpenner