Added a failing MenuItem separator test.

tests/Avalonia.Controls.UnitTests/MenuItemTests.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class MenuItemTests
+    {
+        [Fact]
+        public void Header_Of_Minus_Should_Apply_Separator_Pseudoclass()
+        {
+            var target = new MenuItem { Header = "-" };
+
+            Assert.True(target.Classes.Contains(":separator"));
+        }
+
+        [Fact]
+        public void Separator_Item_Should_Set_Focusable_False()
+        {
+            var target = new MenuItem { Header = "-" };
+
+            Assert.False(target.Focusable);
+        }
+    }
+}