Running PHPUnit tests in DDEV

[finnlewis]

I don't think we have documentation for running tests in ddev yet.

@ekes also pointed to https://github.com/ddev/ddev-selenium-standalone-chrome

So it would be nice to add this and document how to run tests and easily interact with the tests (to figure out why they fail).

[finnlewis]

Also need phpcs and friends in ddev.

[finnlewis]

I'll test this again:

Note: remove headless from:

• MINK_DRIVER_ARGS_WEBDRIVER=["chrome", {"browserName":"chrome","chromeOptions":{"w3c":false,"args":["--disable-gpu","--headless", "--no-sandbox", "--disable-dev-shm-usage"]}}, "http://selenium-chrome:4444/wd/hub"]