Unstable tests

[AArnott]

We should investigate the test failures from the tests disabled in #1040.

SwitchToMainThreadShouldNotLeakJoinableTaskWhenGetResultRunsFirst

Assert.Null() Failure
Expected: (null)
Actual:   Object { }

 at JoinableTaskTests.SwitchToMainThreadShouldNotLeakJoinableTaskWhenGetResultRunsFirst() in D:\a\1\s\test\Microsoft.VisualStudio.Threading.Tests\JoinableTaskTests.cs:line 3462

SwitchToMainThreadMemoryLeak

Assert.Equal() Failure
Expected: TestPassed
Actual:   TestFailed

Stack trace
   at TestUtilities.<>c.<ExecuteInIsolationAsync>b__8_3(Task`1 t) in D:\a\1\s\test\Microsoft.VisualStudio.Threading.Tests\TestUtilities.cs:line 248
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
--- 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)
   at TestBase.ExecuteInIsolation(String testMethodName) in D:\a\1\s\test\Microsoft.VisualStudio.Threading.Tests\TestBase.cs:line 362
   at JoinableTaskTests.SwitchToMainThreadMemoryLeak() in D:\a\1\s\test\Microsoft.VisualStudio.Threading.Tests\JoinableTaskTests.cs:line 3161

SwitchToMainThreadMemoryLeakWithCancellationToken

Assert.Equal() Failure
Expected: TestPassed
Actual:   TestFailed

Stack trace
   at TestUtilities.<>c.<ExecuteInIsolationAsync>b__8_3(Task`1 t) in D:\a\1\s\test\Microsoft.VisualStudio.Threading.Tests\TestUtilities.cs:line 248
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
--- 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)
   at TestBase.ExecuteInIsolation(String testMethodName) in D:\a\1\s\test\Microsoft.VisualStudio.Threading.Tests\TestBase.cs:line 362
   at JoinableTaskTests.SwitchToMainThreadMemoryLeakWithCancellationToken()