Maddie, I am so sorry you're having to go through this! I just did a quick Google search and it looks like it hasn't been proven to be hereditary yet, and there are no tests for breeders but they are working on it. Liver shunt can be aquired after having other liver issues. Here is the information I found:
http://www.roobearyorkies.net/livershunt.cfm
Gina has given good advice. If I were in your situation I would go ahead with the testing too. It's better to be safe than sorry. I hope everything turns out well! Give Lucas some hugs!