Keep custom test logic in one place

test/test-extensions.js

@@ -3,6 +3,12 @@
 // helper functions for async testing and other utility functions.
 (function(global) {
 
+  // show generated maps for rendering tests
+  var showMap = (global.location.search.indexOf('generate') >= 0);
+
+  // show a diff when rendering tests fail
+  var showDiff = (global.location.search.indexOf('showdiff') >= 0);
+
   /**
    * The default tolerance for image comparisons.
    */
@@ -358,7 +364,7 @@
   };
 
   function resembleCanvas(canvas, referenceImage, tolerance, done) {
-    if (window.showMap) {
+    if (showMap) {
       document.body.appendChild(canvas);
     }
 
@@ -373,7 +379,7 @@
         }
 
         if (data.misMatchPercentage > tolerance) {
-          if (window.showDiff) {
+          if (showDiff) {
             var diffImage = new Image();
             diffImage.src = data.getImageDataUrl();
             document.body.appendChild(diffImage);