Benchmark attaching styles.

tests/Avalonia.Benchmarks/Styling/StyleAttachBenchmark.cs

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+using System;
+using System.Runtime.CompilerServices;
+using Avalonia.Controls;
+using Avalonia.UnitTests;
+using BenchmarkDotNet.Attributes;
+
+namespace Avalonia.Benchmarks.Styling
+{
+    [MemoryDiagnoser]
+    public class StyleAttachBenchmark : IDisposable
+    {
+        private readonly IDisposable _app;
+        private readonly TestRoot _root;
+        private readonly TextBox _control;
+
+        public StyleAttachBenchmark()
+        {
+            _app = UnitTestApplication.Start(
+                TestServices.StyledWindow.With(
+                    renderInterface: new NullRenderingPlatform(),
+                    threadingInterface: new NullThreadingPlatform()));
+
+            _root = new TestRoot(true, null)
+            {
+                Renderer = new NullRenderer(),
+            };
+
+            _control = new TextBox();
+        }
+
+        [Benchmark]
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public void AttachTextBoxStyles()
+        {
+            var styles = UnitTestApplication.Current.Styles;
+
+            styles.Attach(_control, UnitTestApplication.Current);
+
+            styles.Detach();
+        }
+
+        public void Dispose()
+        {
+            _app.Dispose();
+        }
+    }
+}