Re: A venerable workhorse
Same way they tested them in the beginning, lots of ground testing, taxi testing and then flight test. 747 had a wicked problem with the initial P&W JT9D-3's as the engine case deformed and caused blades to touch the case, never good thing. Fix was a revised engine mount.
Most twins especially the 767/777 have great single engine performance. Given your doing flight test in a lightly loaded aircraft I would expect a few more air-frames kicking around.