Add a few more tests for number-input

Summary:
Tests for arrows up and arrows down, mostly trying to get a feel for
testing these.

Test Plan: * http://localhost:9000/test

Reviewers: jack, brianmerlob

Reviewed By: jack, brianmerlob

Subscribers: jack, joel

Differential Revision: http://phabricator.khanacademy.org/D8366