Add the ability to select a particular instance from a fixtures file.
Add the ability to select a particular instance from a fixtures file.

This will give us more flexibility in testing performance, by having
more and less complex instances in a props file.

I also removed some statements that were (I believe) left in for
debugging purposes.

Auditors: jlfwong

Test plan:
In webapp: ran `make serve`
In this repo: ran `npm run serve_local` and then `node
tools/benchmark_ka_component.js`.

Everything ran to completion without error.

Khan/react-render-server,Khan/react-render-server,Khan/react-render-server