Fixed DispatcherTimerHelper for unit tests

src/Avalonia.Base/Threading/Dispatcher.Queue.cs

@@ -16,7 +16,10 @@ public partial class Dispatcher
         {
             _backgroundTimer =
                 new DispatcherTimer(this, DispatcherPriority.Send,
-                    TimeSpan.FromMilliseconds(1));
+                    TimeSpan.FromMilliseconds(1))
+                {
+                    Tag = "Dispatcher.RequestBackgroundProcessing"
+                };
             _backgroundTimer.Tick += delegate
             {
                 _backgroundTimer.Stop();

src/Avalonia.Base/Utilities/DispatcherTimerHelper.cs

@@ -12,6 +12,7 @@ public static class DispatcherTimerUtils
     public static void ForceFire(this DispatcherTimer timer)
     {
         timer.Promote();
+        timer.Dispatcher.RemoveTimer(timer);
         timer.Dispatcher.RunJobs();
     }
 }