Restructure source and tests

src/wait.js

@@ -0,0 +1,17 @@
+/**
+ * Wait for a number of milliseconds.
+ *
+ * @param {number} milliseconds The number of milliseconds to wait.
+ * @returns {Promise<string>} Resolves with 'done!' after the wait is over.
+ */
+async function wait(milliseconds) {
+  return new Promise(resolve => {
+    if (isNaN(milliseconds)) {
+      throw new Error('milliseconds not a number')
+    }
+
+    setTimeout(() => resolve('done!'), milliseconds)
+  })
+}
+
+module.exports = { wait }