Unstable test: DequeueCancellationAndCompletionStress

This appveyor build records a test failure:

[xUnit.net 00:00:08.5272512]     Microsoft.VisualStudio.Threading.Tests.AsyncQueueTests.DequeueCancellationAndCompletionStress [FAIL]
[xUnit.net 00:00:08.5274034]       Assert.True() Failure
[xUnit.net 00:00:08.5274584]       Expected: True
[xUnit.net 00:00:08.5275020]       Actual:   False
[xUnit.net 00:00:08.5284068]       Stack Trace:
[xUnit.net 00:00:08.5321563]         C:\projects\vs-threading\src\Microsoft.VisualStudio.Threading.Tests\AsyncQueueTests.cs(419,0): at Microsoft.VisualStudio.Threading.Tests.AsyncQueueTests.DequeueCancellationAndCompletionStress()
Failed   Microsoft.VisualStudio.Threading.Tests.AsyncQueueTests.DequeueCancellationAndCompletionStress
Error Message:
 Assert.True() Failure
Expected: True
Actual:   False
Stack Trace:
   at Microsoft.VisualStudio.Threading.Tests.AsyncQueueTests.DequeueCancellationAndCompletionStress() in C:\projects\vs-threading\src\Microsoft.VisualStudio.Threading.Tests\AsyncQueueTests.cs:line 419

Start with the failure log from the AppVeyor build and the DequeueCancellationAndCompletionStress test in src/Microsoft.VisualStudio.Threading.Tests/AsyncQueueTests.cs, especially line 419. Run the test repeatedly to investigate the intermittent Assert.True failure; done means the stress test passes reliably without reproducing the reported failure.