issue: #3141 add failing tests for grid: when col/row are added grid doesn't trigger layout update

tests/Avalonia.Controls.UnitTests/GridTests.cs

@@ -1472,6 +1472,27 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Fact]
+        public void Add_Column_Should_Invalidate_Grid()
+        {
+            var grid = new Grid { ColumnDefinitions = ColumnDefinitions.Parse("1*,1*") };
+
+            Change_Propery_And_Verify_Measure_Requested(grid, () =>
+            {
+                grid.ColumnDefinitions.Add(new ColumnDefinition(new GridLength(5)));
+            });
+        }
+
+        [Fact]
+        public void Add_Row_Should_Invalidate_Grid()
+        {
+            var grid = new Grid { RowDefinitions = RowDefinitions.Parse("1*,1*") };
+
+            Change_Propery_And_Verify_Measure_Requested(grid, () =>
+            {
+                grid.RowDefinitions.Add(new RowDefinition(new GridLength(5)));
+            });
+        }
         private static void Change_Propery_And_Verify_Measure_Requested(Grid grid, Action change)
         {
             grid.Measure(new Size(100, 100));