The scramble to tighten rules on emissions tests following Volkswagen AG’s diesel-engine scandal is laying bare a system in which car makers pay the very firms that test and certify their vehicles. That system relies on the use of so-called “golden vehicles,” stripped down prototypes that car makers send to testing firms for inspection. The practice, which officials for car makers and testing firms say is widespread, allows car models to undergo tests before they are fitted with everything from back seats to wheels with heavier tread, boosting fuel efficiency and lowering emissions. “It’s like preparing for a major race. We tune them and pamper them like stud horses,” said an executive from one of Europe’s leading auto makers, adding that car companies never submitted random vehicles drawn from the assembly lines for testing. The relationship between car makers and testing firms raises questions about how closely manufacturers can work with the companies without influencing test results. On Wednesday, the French government announced plans to test 100 vehicles drawn directly from car owners and rental agencies. Executives from the auto industry and the testing firms say the use of golden vehicles and other practices don’t amount to cheating, because they aren’t banned by European rules.