Add image_element_properties dom shim to Jest

**Why?**
The mr_widget_options_spec implicitly calls a setFavicon
utility. This method depends on `onload` beind called when
`src` is set.

spec/frontend/helpers/dom_shims/image_element_properties.js

+Object.defineProperty(global.HTMLImageElement.prototype, 'src', {
+  get() {
+    return this.$_jest_src;
+  },
+  set(val) {
+    this.$_jest_src = val;
+
+    if (this.onload) {
+      this.onload();
+    }
+  },
+});

spec/frontend/helpers/dom_shims/index.js

@@ -4,3 +4,4 @@ import './inner_text';
 import './window_scroll_to';
 import './scroll_by';
 import './size_properties';
+import './image_element_properties';