Run tests with Selenium worker again

[borekb]

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.

[borekb]

There's an interesting project called wp-browser that we could consider using.

[borekb]

I'm now convinced that we should probably re-work our Selenium tests to Codeception + wp-browser:

• The phpunit-selenium package is de-facto abandoned, see e.g. PHPUnit 7 supports ? giorgiosironi/phpunit-selenium#422 or no mentions of it in the current PHPUnit docs.
• It pins us to an old version of PHPUnit (6.x; current is 8.x) which has all sorts of unpleasant consequences, for example, I was not able to install other tools: Static code analysis #1451 (comment).
• Codeception seems to worry about real-world challenges around acceptance tests, for example, that calling click on an element might not be enough in JS-heavy applications (I'm quite surprised that they added this only recently in v3.0 but at least they did; we've been fighting these issues ourselves, see things like jsClickAndWait or waitForElement).
• There's the wp-browser package which is even officially suggested on Codeception's website on Codeception's website).
  • I've reached out to wp-browser's author @lucatume via email, he gave me some good tips.
• Respectable WordPress companies like Delicious Brains or 10up use Codeception.

My other thought was to drop Selenium tests entirely but there are scenarios where we still need it, mostly:

• All the tests/Selenium tests, for example, testing that we install .htaccess rules correctly.
• Certain End2End test scenarios cannot be tested via WP-CLI – search for PHPUnit_Framework_SkippedTestError inside *WpCliWorker.php files.

[borekb]

WordPress core now uses Jest + Puppeteer for end2end tests.