Do mechanics that test drive vehicles fall under the hours-of-service rules?

Yes, if they operate commercial motor vehicles (CMVs) in areas that are open to public travel. The federal hours-of-service rules apply to all CMV drivers. A driver is anyone who gets behind the wheel of a “commercial motor vehicle” as defined in 390.5 (or as defined in state regulations for intrastate commerce). Mechanics or other short-haul drivers do not need to fill out a log if they can use either of the short-haul exceptions in 395.1(e).