Unstable test: ExecuteAsync_Contested

The ExecuteAsync_Contested test failed on AppVeyor with the following output:

    ExecuteAsync_Contested(mode: NotAllowed) [FAIL]
      System.Threading.Tasks.TaskCanceledException : A task was canceled.
      Stack Trace:
           at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
           at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
           at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
        src\Microsoft.VisualStudio.Threading.Tests\ReentrantSemaphoreTestBase.cs(164,0): at ReentrantSemaphoreTestBase.<<ExecuteAsync_Contested>b__17_0>d.MoveNext()
        --- End of stack trace from previous location where exception was thrown ---
           at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
           at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
        src\Microsoft.VisualStudio.Threading.Tests\TestBase.cs(310,0): at Microsoft.VisualStudio.Threading.Tests.TestBase.<>c__DisplayClass23_1.<<ExecuteOnDispatcher>b__2>d.MoveNext()
        --- End of stack trace from previous location where exception was thrown ---
        src\Microsoft.VisualStudio.Threading.Tests\TestBase.cs(326,0): at Microsoft.VisualStudio.Threading.Tests.TestBase.<>c__DisplayClass23_0.<ExecuteOnDispatcher>b__0()
        src\Microsoft.VisualStudio.Threading.Tests\TestBase.cs(334,0): at Microsoft.VisualStudio.Threading.Tests.TestBase.ExecuteOnDispatcher(Func`1 action, Boolean staRequired)
        src\Microsoft.VisualStudio.Threading.Tests\ReentrantSemaphoreTestBase.cs(637,0): at ReentrantSemaphoreTestBase.ExecuteOnDispatcher(Func`1 test)
        src\Microsoft.VisualStudio.Threading.Tests\ReentrantSemaphoreTestBase.cs(138,0): at ReentrantSemaphoreTestBase.ExecuteAsync_Contested(ReentrancyMode mode)

Start with ReentrantSemaphoreTestBase.cs at ExecuteAsync_Contested and the reported line 164, then inspect the ExecuteOnDispatcher calls in TestBase.cs at the listed stack-trace locations. Reproduce the AppVeyor failure if possible and trace why TaskCanceledException occurs; done means the test no longer fails intermittently while preserving its intended coverage.