Add missing engine param in package.json (functions/tests) (#457)

Author: rlazo

firebase-functions/src/androidTest/backend/functions/package.json

@@ -8,5 +8,8 @@
   "devDependencies": {
     "@google-cloud/functions-emulator": "1.0.0-beta.4"
   },
-  "private": true
+  "private": true,
+  "engines": {
+    "node": "8"
+  }
 }