function stubNetUtils() {
  stubs.set(netUtils, 'testLoadImage', (url, timeout, callback) => {
    Timer.callOnce(goog.partial(callback, false), timeout);
  });
}
Stubs netUtils to always time out. It maintains the
contract given by netUtils.testNetwork, but always
times out (calling callback(false)).
stubNetUtils should be called in tests that require it before
a call to testNetwork happens. It is reset at tearDown.