Added unit tests for non-disposable DataContext issue (#12123)

tests/Avalonia.LeakTests/DataContextTests.cs

@@ -0,0 +1,70 @@
+using System;
+using Avalonia.Controls;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Reactive;
+using Avalonia.Threading;
+using Avalonia.UnitTests;
+using JetBrains.dotMemoryUnit;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Avalonia.LeakTests;
+
+internal class ViewModelForDisposingTest
+{
+    ~ViewModelForDisposingTest() { ; }
+}
+
+[DotMemoryUnit(FailIfRunWithoutSupport = false)]
+public class DataContextTests
+{
+    public DataContextTests(ITestOutputHelper atr)
+    {
+        DotMemoryUnitTestOutput.SetOutputMethod(atr.WriteLine);
+    }
+    
+    [Fact]
+    public void Window_DataContext_Disposed_After_Window_Close_With_Lifetime()
+    {
+        static IDisposable Run()
+        {
+            var unitTestApp = UnitTestApplication.Start(TestServices.StyledWindow);
+            var lifetime = new ClassicDesktopStyleApplicationLifetime();
+            lifetime.ShutdownMode = ShutdownMode.OnExplicitShutdown;
+            var window = new Window { DataContext = new ViewModelForDisposingTest() };
+            window.Show();
+            window.Close();
+
+            return Disposable.Create(lifetime, lt => lt.Shutdown())
+                .DisposeWith(new CompositeDisposable(lifetime, unitTestApp));
+        }
+
+        using var _ = Run();
+        // Process all Loaded events to free control reference(s)
+        Dispatcher.UIThread.RunJobs(DispatcherPriority.Loaded);
+        GC.Collect();
+
+        dotMemory.Check(m => Assert.Equal(0, 
+            m.GetObjects(o => o.Type.Is<ViewModelForDisposingTest>()).ObjectsCount));
+    }
+    
+    [Fact]
+    public void Window_DataContext_Disposed_After_Window_Close_Without_Lifetime()
+    {
+        static void Run()
+        {
+            using var _ = UnitTestApplication.Start(TestServices.StyledWindow);
+            var window = new Window { DataContext = new ViewModelForDisposingTest() };
+            window.Show();
+            window.Close();
+        }
+
+        Run();
+        // Process all Loaded events to free control reference(s)
+        Dispatcher.UIThread.RunJobs(DispatcherPriority.Loaded);
+        GC.Collect();
+        
+        dotMemory.Check(m => Assert.Equal(0, 
+            m.GetObjects(o => o.Type.Is<ViewModelForDisposingTest>()).ObjectsCount));
+    }
+}