docs: document jest troubleshooting (#382)

example/setupTest.example.js

@@ -0,0 +1,55 @@
+/* eslint-disable import/no-extraneous-dependencies */
+import 'core-js/stable';
+import 'regenerator-runtime/runtime';
+import Enzyme from 'enzyme';
+import Adapter from '@wojtekmaj/enzyme-adapter-react-17';
+import MutationObserver from '@sheerun/mutationobserver-shim';
+import { mergeConfig } from '@edx/frontend-platform';
+
+Enzyme.configure({ adapter: new Adapter() });
+
+/* need to mock window for tinymce on import, as it is JSDOM incompatible */
+
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: jest.fn().mockImplementation(query => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: jest.fn(), // Deprecated
+    removeListener: jest.fn(), // Deprecated
+    addEventListener: jest.fn(),
+    removeEventListener: jest.fn(),
+    dispatchEvent: jest.fn(),
+  })),
+});
+
+mergeConfig({
+  STUDIO_BASE_URL: process.env.STUDIO_BASE_URL,
+  BLOCKSTORE_COLLECTION_UUID: process.env.BLOCKSTORE_COLLECTION_UUID,
+  SECURE_ORIGIN_XBLOCK_BOOTSTRAP_HTML_URL: process.env.SECURE_ORIGIN_XBLOCK_BOOTSTRAP_HTML_URL,
+});
+
+window.MutationObserver = MutationObserver;
+
+let store = {};
+
+const mockStorage = {
+  getItem: (key) => {
+    if (key in store) {
+      return store[key];
+    }
+    return null;
+  },
+  setItem: (key, value) => {
+    store[key] = `${value}`;
+  },
+  removeItem: (key) => {
+    delete store[key];
+  },
+  reset() {
+    store = {};
+  },
+};
+
+Object.defineProperty(window, 'localStorage', { value: mockStorage });