add special_status_message testcase to C# interop client

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

@@ -185,6 +185,9 @@ namespace Grpc.IntegrationTesting
                 case "unimplemented_service":
                     RunUnimplementedService(new UnimplementedService.UnimplementedServiceClient(channel));
                     break;
+                case "special_status_message":
+                    await RunSpecialStatusMessageAsync(client);
+                    break;
                 case "unimplemented_method":
                     RunUnimplementedMethod(client);
                     break;
@@ -567,6 +570,33 @@ namespace Grpc.IntegrationTesting
             Console.WriteLine("Passed!");
         }
 
+        private static async Task RunSpecialStatusMessageAsync(TestService.TestServiceClient client)
+        {
+            Console.WriteLine("running special_status_message");
+
+            var echoStatus = new EchoStatus
+            {
+                Code = 2,
+                Message = "\t\ntest with whitespace\r\nand Unicode BMP ☺ and non-BMP 😈\t\n"
+            };
+
+            try
+            {
+                await client.UnaryCallAsync(new SimpleRequest
+                {
+                    ResponseStatus = echoStatus
+                });
+                Assert.Fail();
+            }
+            catch (RpcException e)
+            {
+                Assert.AreEqual(StatusCode.Unknown, e.Status.StatusCode);
+                Assert.AreEqual(echoStatus.Message, e.Status.Detail);
+            }
+
+            Console.WriteLine("Passed!");
+        }
+
         public static void RunUnimplementedService(UnimplementedService.UnimplementedServiceClient client)
         {
             Console.WriteLine("running unimplemented_service");