Experiment with adding tests for React components

Summary:
This is a first attempt at this, I picked number input because it's
simple enough to understand but it has some somewhat complicated
behaviour (which lets it work well) that would probably benefit from
tests.

There was a small change to the way it grabs the value to make it easier
to test, everything still seems to work fine after that.

Test Plan:
* http://localhost:9000/test
* also check it still works in perseus

Reviewers: joel, jack

Reviewed By: joel, jack

Subscribers: alex, brianmerlob, joel, alpert

Differential Revision: http://phabricator.khanacademy.org/D8356