ui_test_core: fix verifyImageMatchSnapshot with multiple images

The counter was not managed properly, it was never increased, so when
there was more than one assertion, they were all using the same
reference image.

Implement also the TODO that prevented to re-run a test.

bt5/erp5_ui_test_core/SkinTemplateItem/portal_skins/erp5_ui_test_core/user-extensions.js.js

@@ -132,15 +132,17 @@ Selenium.prototype.assertElementPositionRangeTop = function(locator, range){
 };
 
 // a memo test pathname => image counter
-// TODO: reset this on testSuite.reset(), because we cannot re-run a test.
-imageMatchReference = new Map();
+var imageMatchReference = new Map();
+// reset this when starting a new test.
+var HtmlTestRunnerControlPanel_reset = window['HtmlTestRunnerControlPanel'].prototype.reset;
+window['HtmlTestRunnerControlPanel'].prototype.reset = function() {
+  imageMatchReference.clear();
+  return HtmlTestRunnerControlPanel_reset.call(this);
+}
 
 function getReferenceImageCounter(testPathName) {
-  var counter = imageMatchReference.get(testPathName);
-  if (counter !== undefined) {
-    return counter;
-  }
-  counter = imageMatchReference.size + 1;
+  var counter = imageMatchReference.get(testPathName) || 0;
+  counter = counter + 1;
   imageMatchReference.set(testPathName, counter);
   return counter;
 }
@@ -180,7 +182,6 @@ function generateElement(tagName, childList, attributeDict, textContent) {
  *
  * @param {string} referenceImageURL relative URL of the reference image
  * @param {string} newImageData the new image data, base64 encoded
- * @param {Map<string,any>?} attributeDict attributes
  * @return {Promise<string>} the base64 encoded html form
  */
 function generateUpdateForm(referenceImageURL, newImageData) {
@@ -316,6 +317,7 @@ Selenium.prototype.doVerifyImageMatchSnapshot = (
             },
             e => {
               // fetching reference was not found, return empty image instead, it will be different
+              // (unless the tolerance is too high)
               return document.createElement('canvas').toDataURL();
             }
           )