test: add testRender()

This adds a testRender() helper function to simplify UI tests. Instead
of mocking the redux store, we use the real thing.

src/app/App.test.tsx

@@ -1,27 +1,10 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2021 The Pybricks Authors
+// Copyright (c) 2021-2022 The Pybricks Authors
 
-import { I18nContext, I18nManager } from '@shopify/react-i18n';
-import { render } from '@testing-library/react';
-import { mock } from 'jest-mock-extended';
 import React from 'react';
-import { Provider } from 'react-redux';
-import { Store } from 'redux';
-import { RootState } from '../reducers';
+import { testRender } from '../../test';
 import App from './App';
 
 it.each([false, true])('should render', (darkMode) => {
-    const store = mock<Store<RootState>>({
-        getState: () => mock<RootState>({ settings: { darkMode } }),
-    });
-
-    const i18n = new I18nManager({ locale: 'en' });
-
-    render(
-        <Provider store={store}>
-            <I18nContext.Provider value={i18n}>
-                <App />
-            </I18nContext.Provider>
-        </Provider>,
-    );
+    testRender(<App />, { settings: { darkMode } });
 });