Welcome to Yorkie Talk. I'm sorry that Lulu is having problems, and I can understand how scared you must be. You are on the right track, having the vet do testing so you will have a better idea of what you are dealing with.
I want to tell you one important thing up front, having a liver shunt is NOT a death sentence. If there is one, it is treatable, and it may turn out not to be that at all and could possibly be something like microvascular displasia MVD, which can be managed with diet and supplements.
I know the overwhelming fear you have right now, I had that myself when my Tiki was in the same position at the age of 7 months. I almost went crazy as we navigated through the diagnostic process, and in her case a shunt was not found, and they determined she most likely has MVD. I have fed her low protein/low fat diet, and made sure she is not exposed to toxins that her liver cannot process..... and I'm happy to say she is now 6 and doing just fine.
Here are links to some easy to understand information about liver disease that will help you become familiar with the various tests:
http://www.vet.utk.edu/clinical/sacs...NAL-201304.pdf Portosystemic Shunts FAQ
I hope for the best for Lulu, and send hugs to you both. I know it is a heartbreak to have our lil fur kids be sick..... but hang in there, and let's hope this is something that can be fixed.