...an engineer in a flight suit.
...is an FTE?!
Seriously though, I would say that the first requirement of a test pilot is to fly the test with sufficient accuracy to obtain good data and form useful opinion.
To do that safely and effectively, he or she needs to understand how and why that test has been designed; where the edges of safety might be; and how the data will be used. That requires a level of engineering understanding, which (I suspect) is the thrust of Mr Lowry's point. But the TP must still fly the aircraft to the necessary standard in often unusual conditions. This does not normally require an exceptional level of pure handling ability, and the white scarves should most certainly be left at home. But the nature of flight test is that things go wrong, and an ability to react quickly and correctly is also needed.
DAR is right to point out the wide breadth of the flight test world and the range of skill sets needed. But, for me, a test pilot in a flight test team is there as a pilot with engineering understanding. A flight suit with an engineer inside, if you like!