add more relativepanel tests.

tests/Avalonia.Controls.UnitTests/RelativePanelTests.cs

@@ -31,6 +31,7 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Rect(20, 0, 20, 20), target.Children[1].Bounds);
         }
 
+        [Fact]
         public void Lays_Out_1_Child_Below_the_other()
         {
             var rect1 = new Rectangle { Height = 20, Width = 20 };
@@ -55,5 +56,31 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Rect(0, 0, 20, 20), target.Children[0].Bounds);
             Assert.Equal(new Rect(0, 20, 20, 20), target.Children[1].Bounds);
         }
+
+        [Fact]
+        public void RelativePanel_Can_Center()
+        {
+            var rect1 = new Rectangle { Height = 20, Width = 20 };
+            var rect2 = new Rectangle { Height = 20, Width = 20 };
+
+            var target = new RelativePanel
+            {
+                VerticalAlignment = Layout.VerticalAlignment.Center,
+                HorizontalAlignment = Layout.HorizontalAlignment.Center,
+                Children =
+                {
+                    rect1, rect2
+                }
+            };
+
+            RelativePanel.SetAlignLeftWithPanel(rect1, true);
+            RelativePanel.SetBelow(rect2, rect1);
+            target.Measure(new Size(400, 400));
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Size(20, 40), target.Bounds.Size);
+            Assert.Equal(new Rect(0, 0, 20, 20), target.Children[0].Bounds);
+            Assert.Equal(new Rect(0, 20, 20, 20), target.Children[1].Bounds);
+        }
     }
 }