Fix Task test causing CI failures

A Task test is setting a handler for the static UnobservedTaskException event, failing an assert if it's raised. This is problematic if the test runs concurrently with another test that has an unobserved exception. This commit fixes the handler to be more robust and assert only if state related to the one test is incorrect.

Author: stephentoub

src/System.Threading.Tasks/tests/System.Runtime.CompilerServices/AsyncTaskMethodBuilderTests.cs

@@ -434,22 +434,16 @@ public static void RunAsyncAdditionalBehaviorsTests_NegativeCases2()
                 cts.Cancel();
                 b.SignalAndWait(); // release task to complete
 
-                // A TCS task
-                var tcs = new TaskCompletionSource<int>();
-                tcs.SetCanceled();
-                Task t2 = tcs.Task;
-
-                EventHandler<UnobservedTaskExceptionEventArgs> handler = (s, e) =>
-                {
-                    Assert.True(false, string.Format("     > OCE shouldn't have resulted in unobserved event firing with " + e.Exception.ToString()));
-                };
+                // This test may be run concurrently with other tests in the suite, 
+                // which can be problematic as TaskScheduler.UnobservedTaskException
+                // is global state.  The handler is carefully written to be non-problematic
+                // if it happens to be set during the execution of another test that has 
+                // an unobserved exception.
+                EventHandler<UnobservedTaskExceptionEventArgs> handler = 
+                    (s, e) => Assert.DoesNotContain(oce, e.Exception.InnerExceptions);
                 TaskScheduler.UnobservedTaskException += handler;
-
                 ((IAsyncResult)t1).AsyncWaitHandle.WaitOne();
-                ((IAsyncResult)t2).AsyncWaitHandle.WaitOne();
                 t1 = null;
-                t2 = null;
-
                 for (int i = 0; i < 10; i++)
                 {
                     GC.Collect();