I'm not actually in test flight, so perhaps my input is not really useful for this thread, but my few cents (or pennies) worth.
I qualified as an aeronautical engineer, but couldn't find long term work due to a downturn. Did some financial qualifications, went to work for a listed company doing strategic planning, marketing, mergers & acquisitions and so on. Got my PPL, took up aerobatics, bought an aeroplane.
My basic love was still flying, and I still had a desire to get into test work. I spoke to some of the people on this list, got some good advice. Also spoke to a well known local test pilot who gave me some hard but good advice. Firstly, he said that trying to get into test work from outside aviation was pointless. If you are not close to the door, when it opens you won't get through. I took a contract position in aero engineering - same pay, lower perks, but better job satisfaction. After two years the employer then sent me on a short operational test course, was a great learning experience. Most important, a learning experience I would not have got if I had been outside the industry.
The second bit of advice I got was to be careful about academic courses. Experience counts beyond the initial course. He said that test work was a combination of academic and practical. If you can't work with the practical, academics is going to be a waste of time. On the short course I went on, I saw brilliant engineers who struggled to write down a single data point in flight. If I was paying the bills, I'd also demand practical experience from people I hired.
In my case, I don't know what the future holds. Maybe I'll become an FTE, maybe I won't. But I do think the advice I was given was sound.
What do the experts think?