Convert AJAX part of \Drupal\system\Tests\Ajax\MultiFormTest to JavascriptTestBase

Problem/Motivation

drupalPostAjaxForm() is simulating the behaviour of ajax.js, so using it, doesn't really provide fundamental guarantees.
#2809161: Convert Javascript/AJAX testing to use JavascriptTestBase suggests to convert them to JavascriptTestBase

Proposed resolution

1. Figure out which part of the test is testing PHP code and which part ajax behaviour
2. Extract the ajax behaviour into a test that extends JavascriptTestBase

Remaining tasks

User interface changes

API changes

Data model changes