fix typo & add missing attribute

src/Avalonia.Controls/RelativePanel.AttachedProperties.cs

@@ -33,7 +33,7 @@ namespace Avalonia.Controls
             AlignVerticalCenterWithProperty.Changed.AddClassHandler<Layoutable>(OnAlignPropertiesChanged);
             BelowProperty.Changed.AddClassHandler<Layoutable>(OnAlignPropertiesChanged);
             LeftOfProperty.Changed.AddClassHandler<Layoutable>(OnAlignPropertiesChanged);
-            LeftOfProperty.Changed.AddClassHandler<Layoutable>(OnAlignPropertiesChanged);
+            RightOfProperty.Changed.AddClassHandler<Layoutable>(OnAlignPropertiesChanged);
         }
 
         /// <summary>

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 };