fix nit in test

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -1333,11 +1333,9 @@ namespace Avalonia.Controls.UnitTests
 
             public TestDataContext()
             {
-                Items = new ObservableCollection<string>(Enumerable.Range(1, 5).Select(i => GenerateItem()));
+                Items = new ObservableCollection<string>(Enumerable.Range(1, 5).Select(i => $"Item {i}"));
             }
 
-            private string GenerateItem() => $"Item {_counter++.ToString()}";
-
             public ObservableCollection<string> Items { get; }
 
             public string SelectedItem