Improve code coverage

src/app/App.test.tsx

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2021 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 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>,
+    );
+});