Fix and enable a disabled Task test

The test was assuming that if all tasks queued to a scheduler have completed that the scheduler won't be making further use of whatever underlying scheduler it's targeting.  That assumption is false, however, as the scheduler may still queue additional housekeeping tasks, and it requires that the underlying scheduler remain alive until the overlaying scheduler declares completion via its Completion task.

Author: stephentoub

src/System.Threading.Tasks/tests/CESchedulerPairTests.cs

@@ -420,12 +420,8 @@ public static void TestCompletionTask()
 
         /// <summary>
         /// Ensure that CESPs can be layered on other CESPs.
-        /// There is and assert in src\Threading\System\Threading\Tasks\ConcurrentExclusiveSchedulerPair.cs
-        /// line 355 - the scheduler enters in exclusive processing mode when  should not
-        /// it needs more investigations
         /// </summary
         [Fact]
-        [ActiveIssue(2797)]
         public static void TestSchedulerNesting()
         {
             // Create a hierarchical set of scheduler pairs
@@ -490,30 +486,13 @@ public static void TestSchedulerNesting()
                 }
             }
 
-            // Once all tasks have completed, complete the schedulers
-            Task.Factory.StartNew(() =>
-            {
-                Debug.WriteLine(string.Format("Waiting for all tasks", tasks.Count));
-                Task.WaitAll(tasks.ToArray());
-                foreach (var cesp in cesps) cesp.Complete();
-            }, CancellationToken.None, TaskCreationOptions.None, TaskScheduler.Default);
-
-            // In the meantime, wait for all schedulers to complete
-            Debug.WriteLine("Waiting for all csps");
-            foreach (var cesp in cesps) cesp.Completion.Wait();
-
-            Debug.WriteLine("Done waiting");
-            // All tasks should now be done for the schedulers to have completed
-            foreach (var task in tasks)
+            // Wait for all tasks to complete, then complete the schedulers
+            Task.WaitAll(tasks.ToArray());
+            foreach (var cesp in cesps)
             {
-                if (task.Status != TaskStatus.RanToCompletion)
-                {
-                    Debug.WriteLine("Task didn't run to completion");
-                }
+                cesp.Complete();
+                cesp.Completion.Wait();
             }
-
-            // All tasks should now be done for the schedulers to have completed
-            foreach (var task in tasks) Assert.True(task.Status == TaskStatus.RanToCompletion, "All tasks should now be complete");
         }
 
         /// <summary>