Regression test flip feature

[Dan Eble]

Live demo:

    http://faithful.be/tmp/test-results

Go to the images for rest-dot-position and press the "Flip" button under the 
right-hand image.  The new image should change to the old image while the 
button is held.  I've only tested it with the latest Safari for macOS, but I 
hope it works with other recent browsers.

My question for the developers is would you use this if it were available?
— 
Dan