It's one of those mean little facts in life that if you do not gather your NTPS training through the Air Force or Navy, or happen to be rich enough to spend a million at NTPS Mojave, then your chances of being a full-up test pilot for a bizjet manufacturer are almost nil. It can be done though, and I currently know several such persons. Hardly makes sense to spend a million to obtain a $90k job, and but that is the only reliable civvy route. Last I checked NTPS at Mojave was charging a million for their program.
There is another way to get into flight testing, which is to enter it through aero engineering and work your way up from a desk job. That is what I currently do. My job title is Flight Test Engineer, not test pilot. But I am the last line of engineering before the test pilots, and I have no NTPS at all. Anyone can do what I did with a little perseverance and effort. And working as an FTE will quite possibly lead to flying duties in time. Many FTEs fly as often as the test pilots, and are treated as SIC test pilots for all practical matters. All flight time helps in the cause of getting a test pilot job, but a ton of jet time flying 121/91 will not really help. You need the science background to know what to do. That is better served by either test pilot school, or aero engineering as second choice, although far less desirable option.