Amphtml: Visual Diff: Allow `console.log()` in interactive_tests

[torch2424]

Feedback from the visual diff workshop

console.log / console.error doesn't work.

Got a solution working with:

const log = (string) => {
  process.stdout.write(' \n\n');
  process.stdout.write(string);
  process.stdout.write(' \n\n');
};

[danielrozenberg]

I think that log('verbose', ...) in helpers.js should be good enough :) I want to make sure that console log messages aren't printed on Travis, so the verbose log should work for that task