add: unit and integration testing

app/node_modules/babel-preset-jest/index.js

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const jestPreset = {
+  plugins: [require.resolve('babel-plugin-jest-hoist')],
+  presets: [require.resolve('babel-preset-current-node-syntax')],
+};
+
+// @babel/core requires us to export a function
+const jestPresetPlugin = () => jestPreset;
+module.exports = jestPresetPlugin;