Add manual mock

tests/javascript/mochaTestHelper.js

@@ -97,6 +97,15 @@ global.stubImage = function (defaultWidth, defaultHeight) {
   };
 };
 
+global.window.matchMedia = window.matchMedia || (
+  () => {
+    return {
+      matches: false,
+      addListener: () => {},
+      removeListener: () => {},
+    };
+  }
+);
 
 testHelpers.loadTemplate('blocks/base/toolsGeneric.hbs', window, { id: 'newsletter_editor_template_tools_generic' });