It's likely a long path any way you go. You usually need to be a pretty darn good engineer, and a pretty darn good pilot. Possible shortcuts involve the military and eventually getting sent to test-pilot school. I would still stress how important it is to be a good engineer though. Often times the few that make it into test pilot school are part of the small cadre of civilian test-pilots employed by the big manufacturers like boeing and others. When they aren't, it's the types that have been successful at both with lots of hours and experience under their belt. Civilians AND military pilots get sent to "test pilot school", but the cost is usually covered by the sponsoring entity (it's bloody expensive, like million or more invested in you during the school). People usually only get to that spot after years of experience and training elsewhere.
Really think this one over. Maybe flying P8s (recently mentioned) AND having an EE degree (think about electronic warefare) would be a good way to go, because then you get boeing time and a shot at working for them when you get out, maybe in engineering, maybe in piloting, maybe get hired at a lower level or field not as related and then work your way there. No one is going to do this fast, you're likely going to reach "test pilot" status when you are getting kind of old (like most good things in life
), but really think about how you might go about this. Don't stack things up against you like not getting an engineering degree, or maybe competing for flighter jet slots when they are dwindling down, not to say that is necessarily a bad route, just make sure to think it over.