Add Javascript tests for Form API's #states: require, visible, invisible

Problem/Motivation

Form API's integration with javascript through #states is extremely complex. Now we have JavascriptTestBase we should test this.

Proposed resolution

Add copious tests and fix bugs found.

We can get inspiration for tests from:

• https://www.drupal.org/project/examples
• https://www.lullabot.com/articles/form-api-states

Remaining tasks

User interface changes

API changes

Data model changes