Add example testCredentials object since the format is not easily understood using the link.

Testing.md

@@ -67,6 +67,17 @@ Helper script can start Node-RED server, load a flow, and receive a payload from
 - `testNode`: (object|array of objects) Module object of a node to be tested returned by require function. This node will be registered, and can be used in `testFlows`.
 - `testFlows`: (array of objects) [Flow data](https://github.com/node-red/node-red/wiki/Flow-Format) to test a node. If you want to use the flow data exported from Node-RED editor, need to covert it to JavaScript object using JSON.parse().
 - `testCredentials`: (object) [Node credentials](https://nodered.org/docs/creating-nodes/credentials). This argument is optional.
+
+Example `testCredentials` for node "n1":
+```
+    {
+        n1: {
+            user: "user",
+            password: "password"
+        }
+    }
+```
+
 - `cb`: (function) Function to call back when testFlows has been started.
 
 The minimum script to test a flow is as follows. This case omits `testCredentials`.