Fake SPContext as a particular user

Question

I am using this method of faking an SPContext: http://solutionizing.net/2009/02/16/faking-spcontext/

public static SPContext FakeSPContext(SPWeb contextWeb)
{
  // Ensure HttpContext.Current
  if (HttpContext.Current == null)
  {
    HttpRequest request = new HttpRequest("", web.Url, "");
    HttpContext.Current = new HttpContext(request,
      new HttpResponse(TextWriter.Null));
  }

  // SPContext is based on SPControl.GetContextWeb(), which looks here
  if(HttpContext.Current.Items["HttpHandlerSPWeb"] == null)
    HttpContext.Current.Items["HttpHandlerSPWeb"] = web;

  return SPContext.Current;
}

Now, if I need this context to have identity of a specific SharePoint user. Any ideas how would I do that?