TestSendAsync_DelayedConsume test failed in CI

[stephentoub]

06:21:05    System.Threading.Tasks.Dataflow.Tests.DataflowBlockExtensionsTests.TestSendAsync_DelayedConsume [FAIL]
06:21:05       Assert.False() Failure
06:21:05       Expected: False
06:21:05       Actual:   True
06:21:05       Stack Trace:
06:21:05             at System.Threading.Tasks.Dataflow.Tests.DataflowBlockExtensionsTests.<TestSendAsync_DelayedConsume>d__28.MoveNext()
06:21:05          --- End of stack trace from previous location where exception was thrown ---
06:21:05             at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
06:21:05             at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
06:21:05          --- End of stack trace from previous location where exception was thrown ---
06:21:05             at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
06:21:05             at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
06:21:05          --- End of stack trace from previous location where exception was thrown ---
06:21:05             at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
06:21:05             at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)