improve test

tests/Avalonia.Controls.UnitTests/ListBoxTests.cs

@@ -321,6 +321,9 @@ namespace Avalonia.Controls.UnitTests
 
                 items.Insert(2, "2+");
 
+                lm.ExecuteLayoutPass();
+                //after few more layout cycles layoutmanager shouldn't hold any more visual for measure/arrange
+                lm.ExecuteLayoutPass();
                 lm.ExecuteLayoutPass();
 
                 var flags = System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic;