Spike: Look at how to support component-level tests

I've spent some time trying to make importing from `wp-calypso` and `gridicons` possible in tests, but to no avail so far. It's a tall order because of the multiple roots in the webpack resolve. This is difficult to make happen with `babel-register` as we're using it now.

The way I see it, here are our options:
1. Run mocha from webpack (difficulties: having mocha iterate through the tree, looking for tests)
2. Add a custom `resolveModuleSource` for `babel-register` that can check multiple source roots.
3. Get us to a single source root by changing all the imports to components.

Also, I've seen this https://github.com/trayio/babel-plugin-webpack-alias which is pretty close to what we need, but it needs to support `resolve.root` for what we want to do to work.

Update: Use shallow rendering
http://slides.com/nordnetacademy/testing-react-apps#/3/8
https://facebook.github.io/react/docs/test-utils.html

Start by reviewing the current babel-register and Mocha setup, then trace how webpack resolves the multiple source roots for wp-calypso and gridicons. Compare the three listed approaches and the shallow-rendering update; the work is done when a concrete approach for component-level tests is selected and its feasibility is established.

Written by the indexing model from the issue text.