WPT: attribute and node removal scenarios

[jasonwilliams]

We've updated the spec to deal with the containertiming attribute being removed and the node itself being removed from the DOM, but there are no WPT tests for this, we should track the updating of the test suite to cover this scenario

[jasonwilliams]

Some examples:

• One is node removal in container timing, and what happens if the reported container timing refers to it the field should be empty, it should show null because that element doesn't exist anymore and its a weak pointer.

• When we remove the attribute, we should be clearing the database for the container paints of that element, I'm not sure how we would write tests for this.