Akka.Hosting.TestKit: added NullScope to xUnitLogger (#570)

This prevents a `NotImplementedException` from being thrown inside some test cases when working with other components that depend on scopes via Microsoft.Extensions.Logging

Author: Aaronontheweb

src/Akka.Hosting.TestKit/Internals/XUnitLogger.cs

@@ -70,7 +70,7 @@ public bool IsEnabled(LogLevel logLevel)
 
         public IDisposable BeginScope<TState>(TState state) 
         {
-            throw new NotImplementedException();
+            return NullScope.Instance;
         }
 
         private static bool TryFormatMessage<TState>(
@@ -91,6 +91,13 @@ private static bool TryFormatMessage<TState>(
             result = formattedMessage;
             return true;
         }
+
+        private class NullScope : IDisposable
+        {
+            private NullScope() { }
+            public static NullScope Instance { get; } = new NullScope();
+            public void Dispose() { }
+        }
     }    
 }