ui_test_core: add waitForFontsLoaded command

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

@@ -324,3 +324,12 @@ Selenium.prototype.doVerifyImageMatchSnapshot = (
       }));
 
 };
+
+/**
+ * Wait for fonts to be loaded.
+ *
+ * This is useful before checking snapshot
+ */
+Selenium.prototype.doWaitForFontsLoaded = function() {
+  return wrapPromise(this.page().getDocument().fonts.ready);
+}
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