The issue is not about putting a "qualified pilot" at the controls during engine runs, the issue is ensuring that those at the controls have the necessary training to carry out those tasks and that they follow the procedures necessary for the task concerned. Start shortcutting procedures and sooner or later, something will happen. You would be amazed how much different a high power engine run is from an aircraft during a takeoff run. I can't comment on a 340 engine run, but I can assure you 320's and 767's bounce all over the place, and thats only two engines. In a restricted run bay such as at Toulouse (I've been there so I know what it's like) you have very little time to react if something is going wrong.
The biggest problem a lot of engineers face is that a lot of companies don't want to train us in simulators. We learn "on the job". So training to recognise things like brake failure and aircraft movement after jumping chocks may never be carried out. It's always in the back of your mind if you are aware of it's possibility. It all comes down to being competent at what you are doing, following procedures and maintaining situational awareness. Some of these things can never be learnt out of a book.