Jest configuration fix (#15673)

test/jest/setup.js

@@ -1,2 +1,10 @@
 // This file is for Jest-specific setup only and runs before our Jest tests.
 import '@testing-library/jest-dom';
+
+jest.mock('webextension-polyfill', () => {
+  return {
+    runtime: {
+      getManifest: () => ({ manifest_version: 2 }),
+    },
+  };
+});