When you ask about the duties of a test engineer, the answers fall into one of two categories: creating test stations and maintaining test stations. Many times these duties will be handled by the same person/group - sort of a "you built it, you own it" theory. I've been in several of those situations before. I have also worked for places where the test system was conceived in the R&D group and then turned over to production. Production modified it and maintained it. In that case the person maintaining the system was a sustaining engineer.
I found a Dept. of Defense definition of sustaining engineering as follows:
The technical effort to support an in-service system in its operational environment. This effort spans those technical tasks (engineering and logistics investigations and analyses) to ensure continued operation and maintenance of a system with managed (i.e. known) risk.
I would consider this to be a fairly accurate summary. So, the point of my post is "how much of this is test engineering and how much is it manufacturing engineering?" The sustaining engineer is not creating a test system, but he is responsible for a system's upkeep, making improvements if needed, and monitoring/analyzing the data it creates. These duties are directly related to the test system. But in theory the test system should have been engineering well enough so that it can be turned over to manufacturing without a lot of care and feeding. I mean, Agilent may build a $200k test system that they sell, but they do not sell an engineer with that system.
Personally, I think the answer is that the sustaining engineer is part of the test department (if the dept is large enough to handle that subdivision of labor). He may have had a hand in the creation of the system, and now he helps run it. Ideally, he still has time to work on other projects because the test station has few problems.
Of course, if your test group primarily does R&D work, then all bets are off. Any test system you build will probably be used a few dozen times and then scrapped in favor of something new...