[QueueInput] will normally use JSON.Net to deserialize the queue message payload to the parameter type. So if the queue message is not JSON, you'll get a exception (which should then be wrapped in something more friendly).
You can also work around it by using a string parameter with [QueueInput], like:
public static void Function([QueueInput] string testqueue)
For string parameter, the SDK will give you the QueueMessage.AsString directly, without any JSON serialization.
FYI, $AzureJobsParentId is a special field placed on json payloads that identifies which function instance enqueued a message. This gets used when you enqueue a message with [QueueOutput]. You can then view that relationship in the SDK dashboard (http://blogs.msdn.com/b/jmstall/archive/2014/01/27/getting-a-dashboard-for-local-development-with-the-webjobs-sdk.aspx)