Rework the third section of the mix-blend-mode test plan

Rework section 5.1.3 from the blending test plan found at http://mire.github.io/css-blending-test-plan-proposal/css-blending-test-plan-proposal.html#an-element-with-mix-blend-mode-doesn-t-blend-with-anything-outside-the-current-stacking-context
Reviewer: Mihai Balan