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.
public void CheckWorkflowWriteLine()
var wi = new WorkflowInvoker(new HelloWorld());
//Add string writer to extensions
var writer = new StringWriter();
Assert.AreEqual("Hello Workflow", writer.ToString());