Avoid using dangerous local variable in test

osu.Framework.Tests/Threading/ThreadedTaskSchedulerTest.cs

···
         [Test]
         public void EnsureEventualDisposalWithStuckTasks()
         {
-            bool running = true;
-
             ManualResetEventSlim exited = new ManualResetEventSlim();
 
             Task.Run(() =>
···
                 {
                     Task.Factory.StartNew(() =>
                     {
-                        while (running)
+                        while (!exited.IsSet)
                             Thread.Sleep(100);
                     }, default, TaskCreationOptions.HideScheduler, taskScheduler);
                 }
···
             });
 
             Assert.That(exited.Wait(30000));
-            running = false;
         }
     }
 }