Javascript end-to-end testing

Our Drupal brethren have gone through a series of tasks over the years of the D8/D9 cycle, each one introducing a new way to test JS, and deprecating the previous way:

• Javascript end-to-end testing added (JavascriptTestBase), using Phantomjs (introduced in D8.1 / change record / https://www.drupal.org/project/drupal/issues/2469713 / https://www.drupal.org/project/drupal/issues/2702661)
• Test Drupal using webDriver/selenium/chromedriver (introduced in D8.5 / change record / https://www.drupal.org/project/drupal/issues/2775653)
• JavascriptTestBase deprecated in favor of WebDriverTestBase (change record / https://www.drupal.org/project/drupal/issues/2942900 / https://www.drupal.org/project/drupal/issues/3088688)

It is my understanding that there is no JS testing in D7/Backdrop, and that we rely on manual testing when it comes to it. This issue here is to discuss and decide whether we should introduce a framework to be testing JS in Backdrop.

Pros

• no more tedious/manual tests for JS functionality
• leverage any tests already in D8+ that also apply to us (like tests for #states for example: #5161)
• ...?

Cons

• introducing dependencies to run tests
• increase in time for automated tests to run
• ...?