Document how to test the WebDriver integration

[jdm]

I have no idea how to go about this yet. Assigning to @jgraham who has the most experience here; I'd like to sort this out soon so stuff like #5554 isn't flying blind.

[jgraham]

Right, so, "it's a mess".

web-platform-tests has a webdriver testsuite, but it's not great and may fail horribly if you don't already have a good implementation. It also isn't integrated with wptrunner. I started on a better testsuite in a local branch, that integrates with wptrunner, but so far didn't really write many tests. I would encourage anyone working on this to contribute to that.

In reality what everyone (except me!) cares about is interop with selenium, which has its own testsuite. However the WebDriver spec and selenium aren't directly compatible, so to run the selenium testsuite you need a fork of selenium from @AutomatedTester which has a modified client and testsuite.