A couple things about our (UA) test, tech and engineering folks. They do much more than just tests after heavy checks. They also fly air shows as well as assist OEMs with their test campaigns (we are one of only a couple airlines in the world that do this). Additionally, any post-maintenance flights are required to be put into open time for line folks to pick up, but since we are not trained for those flights, we are immediately bought off and they are assigned to the tech folks. It can be pretty lucrative because the planes that go to Asia for heavy checks are often routed on 6 or 7 day trips. Occasionally getting the planes to or from maintenance will be flown by line pilots, but not the actual post-maintenance check flights.