Engineers tend to go on test flights when there is requirement to ie. tracking and balancing or a specific problem that needs to be understood/helped/solved by them. Engineers are usaully busy with many aircraft and when a single/dual pilot udertakes an airtest, it is usually because of this. Afterall, flying these machines is what the pilot is paid for.
Before an aircraft even leaves the hangar, especially after a major component change or servicing, it is subject to a lot of detailed inspections. These include duplicate and full and free functional checks and a lot of paperwork. Once outside it is subject to ground runs for leak and functional checks of the systems. When and only when it's serviceability has been proven, will the aircarft be released to service or a subsequent air test. This is what an engineer is paid for and rest assured you are well looked after.
It takes many years of dedication and training to produce a competant engineer, good pilots know their worth. Sadly, some company's don't
..apprenticeships anyone?
In general,engineers and pilots have a good working relationship and a mutual respect for one another, i enjoy the banter with them. Human factors can bite at anytime, be it in the form of pilot error, maintenance error or a manufacturers error. It is what the industry learns from these terrible accidents and what steps it is prepared to make in preventing them, that i find more concerning.