Add tests for AjAX Example

Since #789186: Improve drupalPostAJAX() to be used more easily, leading to better AJAX test coverage we've had the ability to create tests for AJAX operations. It's not a true browser test, but it is a test, and would help us quite a lot.

This would not only help to stabilize the AJAX Examples, but would also provide an example of how to write AJAX-type tests.

There is one example that I know of, #926016: Several bugs when trying to remove files from a multivalued file/image field.