Add JS functional tests for each untested form in ajax_example

Problem/Motivation

This is a follow-up to #2102639: Port ajax_example module to Drupal 8

The AJAX Example module does due dilligence to make sure the forms render, but it does not check interaction.

Each example form that does not have a functional JavaScript test should have one.

Proposed resolution

Add a functional JavaScript test for each example form.

Remaining tasks

User interface changes

API changes

Data model changes