Workflow background thread - how to wait

Question

I have a persistent workflow (runned by WorkflowApplication class) and I need to unit test correctness of runnung this workflow.

I am running workflow from unit test and all runs fine, except I receive error message in output window:

System.AppDomainUnloadedException: Attempted to access an unloaded AppDomain. This can happen if the test(s) started a thread but did not stop it. Make sure that all the threads started by the test(s) are stopped before completion.

I want to test results of workflow run after it was persisted and unloaded, so I use code like this:

AutoResetEvent waitHandle = new AutoResetEvent(false);
WorkflowApplication wfApp = new WorkflowApplication(workflowTypeInstance, inputs);
wfApp.Unloaded = delegate(WorkflowApplicationEventArgs e)
{   
    waitHandle.Set();
};
// ....
wfApp.Run();
waitHandle.WaitOne();

The problem is, that workflow application background thread does not terminate right after unload of workflow, but test method does. This causes the error message in output window.

Any idea / approach / point how to wait for background thread termination?