Added InputPaths API tests for getInputPaths and getDOMNodeForInputPath

Summary:
Added tests for some of the new item-renderer input-related API functions.
I also moved the test item JSON objects into a separate directory to avoid bloating
the renderer-api-test.jsx file. #fixit

Test Plan: `make test`

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D12079