add more unit tests for relative panel

tests/Avalonia.Controls.UnitTests/RelativePanelTests.cs

@@ -6,13 +6,15 @@ namespace Avalonia.Controls.UnitTests
     public class RelativePanelTests
     {
         [Fact]
-        public void Lays_Out_1_Child_Below_the_other()
+        public void Lays_Out_1_Child_Next_the_other()
         {
             var rect1 = new Rectangle { Height = 20, Width = 20 };
             var rect2 = new Rectangle { Height = 20, Width = 20 };
 
             var target = new RelativePanel
             {
+                VerticalAlignment = Layout.VerticalAlignment.Top,
+                HorizontalAlignment = Layout.HorizontalAlignment.Left,
                 Children =
                 {
                     rect1, rect2
@@ -20,6 +22,31 @@ namespace Avalonia.Controls.UnitTests
             };
 
             RelativePanel.SetAlignLeftWithPanel(rect1 , true);
+            RelativePanel.SetRightOf(rect2, rect1);
+            target.Measure(new Size(400, 400));
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Size(40, 20), target.Bounds.Size);
+            Assert.Equal(new Rect(0, 0, 20, 20), target.Children[0].Bounds);
+            Assert.Equal(new Rect(20, 0, 20, 20), target.Children[1].Bounds);
+        }
+
+        public void Lays_Out_1_Child_Below_the_other()
+        {
+            var rect1 = new Rectangle { Height = 20, Width = 20 };
+            var rect2 = new Rectangle { Height = 20, Width = 20 };
+
+            var target = new RelativePanel
+            {
+                VerticalAlignment = Layout.VerticalAlignment.Top,
+                HorizontalAlignment = Layout.HorizontalAlignment.Left,
+                Children =
+                {
+                    rect1, rect2
+                }
+            };
+
+            RelativePanel.SetAlignLeftWithPanel(rect1, true);
             RelativePanel.SetBelow(rect2, rect1);
             target.Measure(new Size(400, 400));
             target.Arrange(new Rect(target.DesiredSize));