An employer doesnt have to be some 'right-wing religious nut', or spend large sums of dough to have the lab test for a non-human urine sample - the labs do it anyway as part of the validity check.
A "non-human" sample is determined when the creatinine level is between zero and 2 mg/dL. As long as the synthetic is of a reputable brand, is not expired, and hadnt been exposed to any direct sunlight, you have nothing to worry about in the validity check department.

You can actually perform your own validity test on the synthetic sample to ensure that the creatinine, pH, and specific gravity are all within acceptable ranges. Do a Google search for drug adulteration test kits.