I find it interesting that several airlines seem to have found crew to fly these test flights and repositioning flights, but the courier companies (UPS etc) still seem grounded. Even KLM are apparently resuming cargo-only flights. What would be the reason that UPS have not found someone to fly their planes?
There is a very good chance that engines won't fail during a test flight. The danger is that they will fail within a hundred hours or so; not only that there could be a multiple failure as they would all have a similarly reduced life span.
Customers will soon come to their own conclusions and avoid flying in any case.