We currently only run end2end tests with the WP-CLI driver, see here, and the other Selenium tests have been disabled in dc531ed (PR #1329) as it has been challenging to make them work. It has been postponed for post-beta work in #1284 but it's worth a separate issue so I'm creating this one.

The PR #1329 mentions this around Selenium tests:

Make Selenium tests work again. I spent some time on them in this PR but they will require more love. Quick thoughts at this point:

• The actual test code (navigating URLs, clicking on things) still looks fine to me but the setup code is slightly suspicious, e.g., here or here.
• facebook/webdriver seems to be more popular than giorgiosironi/phpunit-selenium (Selenium2TestCase), I've also heard good things about Codeception. As long-term maintainability of our Selenium tests is an important goal, considering a new framework is something we should do.

Also related is the ability to switch WP-CLI and Selenium workers more dynamically than via test-config.yml – we don't have a separate issue for that yet but it's something I'd like to do as part of #1389 or shortly after that.