Here is how you get your response as string:
var responseString = response.Content.ReadAsStringAsync().Result;
However json format can vary and I bet you don't want to test that - so I recommend using Newtonsoft.Json or some similar library, parse the string to json object and test json object properties instead. That'll go
using Newtonsoft.Json.Linq;
dynamic jsonObject = JObject.Parse(responseString);
int status = (int)jsonObject.Status;
Assert.Equal(0, status);