Actually dispose DisposeAnimationInstanceSubject.

We were creating a `DisposeAnimationInstanceSubject` and then just discarding it, meaning it was never disposed.

Fixes #6111
Fixes #9514

src/Avalonia.Base/Animation/Animators/Animator`1.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using Avalonia.Animation.Utils;
 using Avalonia.Collections;
@@ -39,7 +40,7 @@ namespace Avalonia.Animation.Animators
                 VerifyConvertKeyFrames();
 
             var subject = new DisposeAnimationInstanceSubject<T>(this, animation, control, clock, onComplete);
-            return match.Subscribe(subject);
+            return new CompositeDisposable(match.Subscribe(subject), subject);
         }
 
         protected T InterpolationHandler(double animationTime, T neutralValue)

tests/Avalonia.Base.UnitTests/Animation/AnimationIterationTests.cs

@@ -181,7 +181,7 @@ namespace Avalonia.Base.UnitTests.Animation
             Assert.Equal(border.Width, 300d);
         }
 
-        [Fact(Skip = "See #6111")]
+        [Fact]
         public void Dispose_Subscription_Should_Stop_Animation()
         {
             var keyframe1 = new KeyFrame()
@@ -310,7 +310,7 @@ namespace Avalonia.Base.UnitTests.Animation
             Assert.True(animationRun.IsCompleted);
         }
 
-        [Fact(Skip = "See #6111")]
+        [Fact]
         public void Cancellation_Should_Stop_Animation()
         {
             var keyframe1 = new KeyFrame()
@@ -372,7 +372,6 @@ namespace Avalonia.Base.UnitTests.Animation
             clock.Step(TimeSpan.FromSeconds(1));
             clock.Step(TimeSpan.FromSeconds(2));
             clock.Step(TimeSpan.FromSeconds(3));
-            //Assert.Equal(2, propertyChangedCount);
 
             animationRun.Wait();