Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/test/helpers/map-values.js

@@ -0,0 +1,7 @@
+function mapValues(obj, fn) {
+  return Object.fromEntries([...Object.entries(obj)].map(([k, v]) => [k, fn(v)]));
+}
+
+module.exports = {
+  mapValues,
+};