I found an article from late 2013 about the inheritance of liver shunt and elevated bile acids in Maltese dogs. The work is very preliminary, so it looks like a genetic test is still a ways away. The article is behind a paywall, but here is a link:
The inheritance of extra-hepatic portosystemic shunts and elevated bile acid concentrations in Maltese dogs - O'Leary - 2013 - Journal of Small Animal Practice - Wiley Online Library
Quoting directly from the article, the main conclusions are:
This study represents a preliminary analysis into the inheritance of EHPSS [Extra-hepatic Portosystemic Shunt] in Maltese. While it provides evidence for the genetic transmission of the condition, future work would ideally recruit additional cases and archive DNA to permit whole genome sequencing or genome-wide association studies using canine single nucleotide polymorphism microarrays to identify loci associated with EHPSS and/or PVH [portal vein hypoplasia] in this breed. Indeed, such studies have commenced in Utrecht and Cornell concentrating on dachshunds, Cairn and Yorkshire terriers (van Steenbeek et al. 2012; J. Rothuizen, personal communication). The impact of identified genes on the stunting of the terminal portal veins and regeneration following shunt attenuation would be informative (Furneaux 2011). Definitive identification of PVH with a readily available and minimally invasive test (as opposed to multiple liver biopsies using laparoscopy) would be very helpful as a screening tool, and protein C determinations (Toulza et al. 2006) may achieve this end if used in concert with RATT or advanced imaging. In summary, PSS is inherited in Maltese possibly with a common, partially penetrant, single locus model. Further work, including molecular studies, may help provide further information on inheritance, genetic causes and aetiopathogenesis of EHPSS. This is germane, as prevention is clearly a superior option to surgical correction.