Disable ClientWebSocketTest.cs (for mobile) until the type itself is added to the mobile profile (i.e. added to mobile_System.dll.sources). Fix building/running System.dll unit tests

mcs/class/System/Test/System.Net.WebSockets/ClientWebSocketTest.cs

@@ -1,4 +1,4 @@
-#if NET_4_5
+#if NET_4_5 && !MOBILE
 using System;
 using System.Net;
 using System.Threading;