Visual regression testing for OPT frontend
started exploring (again!!!) on 2018-06-09 using Google Puppeteer (https://pptr.dev/)

this is (at least) my 3rd attempt at this: {

  for a REALLY old prior attempt at frontend visual regression testing
  using phantomjs (created around 2013-11-23 and updated in early 2014), see:
    ../../../v3/tests/frontend-tests/opt-screenshot-test.js

  and for a less old prior attempt from July 2016 using casperjs, see:
    ../../../tests/frontend-regression-tests/

}

Dependencies: see top of frontend-golden-test.js
