More Dispatcher.UIThread.RunJobjs() in tests

tests/Avalonia.LeakTests/AvaloniaObjectTests.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Reactive.Subjects;
+using Avalonia.Threading;
 using JetBrains.dotMemoryUnit;
 using Xunit;
 using Xunit.Abstractions;
@@ -56,7 +57,9 @@ namespace Avalonia.LeakTests
 
             completeSource();
             GC.Collect();
-
+            // Forces WeakEvent compact
+            Dispatcher.UIThread.RunJobs();
+            GC.Collect();
             Assert.False(weakSource.IsAlive);
         }