Factor out renderTests, move TodoMVC tests into example

* all DRT tests now refer to "packages/web_ui/testing/testing.js"
* TodoMVC tests and baselines move to example/todomvc/test
* moved DRT logic from run_all and run.sh into lib/testing/render_test.dart
* the README and build file and pubspec now match editor's copy

The goal here is to get TodoMVC self-contained, so we can run automated tests to validate the sample that ships in the Editor.

Review URL: https://chromiumcodereview.appspot.com//12902025