Created new test for client connection failed

Sharp7.Tests/ClientWithoutServer.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Linq;
+using Shouldly;
+using Xunit;
+
+namespace Sharp7.Tests
+{
+    public class ClientWithoutServer
+    {
+        private S7Client client;
+
+        public ClientWithoutServer()
+        {
+            client = new S7Client();
+        }
+
+        public new void Dispose()
+        {
+            client.Disconnect();
+        }
+
+        [Fact]
+        public void CannotConnectTest()
+        {
+            var rc = client.ConnectTo("127.0.1.2", 0, 2);
+            rc.ShouldBe(Sharp7.S7Consts.errTCPConnectionFailed);
+        }
+    }
+}