Better Drupal setup for testing

[jhedstrom]

It's relatively easy to now properly use composer for setting up Drupal 8 during testing.

    "require-dev": {
        "composer/installers": "^1.2",
        "drupal-composer/drupal-scaffold": "2.3.0",
        "drupal/core": "8.5.x-dev",

this will allow for easier testing of #157 since that includes tests that require Drupal.

[jonathanjfshaw]

I was inspired by your work here to work on this for #157, now updated. But I ended up needing to take a much more complex tack similar to what the extension does. But I organised it neatly :)