Tagging all instructions to human testers.

As a part of making the WebRTC tests run as Blink layout tests (https://crbug.com/389134) I'm tagging the manual instructions in the tests with a special class "instructions", as agreed upon with dom@w3.org. Tags with this class will rewritten during import to the layout tests so they're invisible (style="display: none"). This way the test runner will not render the instructions and we don't have to create an expectations file containing only the instructions. Only the testharness output will be printed, which is parsed correctly already by the test runner.