Added failing ContextMenu test.

Due to NRE introduced in #3745.

tests/Avalonia.Controls.UnitTests/ContextMenuTests.cs

@@ -1,9 +1,5 @@
 using System;
-using System.Windows.Input;
-using Avalonia.Controls.Primitives;
-using Avalonia.Data;
 using Avalonia.Input;
-using Avalonia.Markup.Data;
 using Avalonia.Platform;
 using Avalonia.UnitTests;
 using Moq;
@@ -159,6 +155,19 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Can_Set_Clear_ContextMenu_Property()
+        {
+            using (Application())
+            {
+                var target = new ContextMenu();
+                var control = new Panel();
+
+                control.ContextMenu = target;
+                control.ContextMenu = null;
+            }
+        }
+
         [Fact(Skip = "The only reason this test was 'passing' before was that the author forgot to call Window.ApplyTemplate()")]
         public void Cancelling_Closing_Leaves_ContextMenuOpen()
         {