FWIW, My experience, and that of many test pilots I know, has been that they started in a non flying role in the company, and in the mean time, building piloting skills independantly. Often the beginning point was one of maintenance or production on the company's aircraft.
For my experience, a few thousand hours of GA flying in many types, coupled with maintenance check flying, and sale demonstration flying was the starting point. A non flying Transport Canada delegation for aircraft certification (issuing STC's) gave me the value to clients for STC test flight work.
But, for me, there's not enough work to make a living at it alone, much less pay the cost of a comprehensive flight test course. My training has been short courses and peer mentoring only, specific to what I flight test.