Unstable tests

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()

Start with the disabled tests named in #1040 in test/Microsoft.VisualStudio.Threading.Tests/JoinableTaskTests.cs, then run them to reproduce the failures. Read the referenced TestUtilities.cs and TestBase.cs isolation paths, along with the failure locations at lines 3462 and 3161. Done means the three tests pass reliably without the reported assertion or isolation failures.