If you wish to test specific writelines on a console application: you can do so by adding a StringWriter object to the workflow extensions.
The writeline activity will use the textwriter if it exists as opposed to the console.

[TestMethod]
public void CheckWorkflowWriteLine()
{   
 var wi = new WorkflowInvoker(new HelloWorld());

        //Add string writer to extensions  
 var writer = new StringWriter();    
 wi.Extensions.Add(writer);    
 wi.Invoke();    
 Assert.AreEqual("Hello Workflow", writer.ToString());
}