More than a decade ago, the U.S. Environmental Protection Agency helped develop a technology that ultimately was used by an independent laboratory to catch Volkswagen’s elaborate cheating on car emissions tests. But EPA used the technology primarily to test trucks rather than passenger cars because such heavy equipment was a much bigger polluter.That decision meant that the U.S. regulator missed its best chance to foil the German carmaker’s deception early on.The portable emissions measurement systems that EPA …