If you can find employment as a flight test intern, certainly take it! Be where the flying and testing is happening. That said, generally flight test organizations are looking for people who have formal flight test training. There are a number of flight test schools around the world. Many cater mostly to the military, and yes, civil flight test staff are often retired military who have taken those courses during their military service. Because of the cost to operate advanced aircraft, these courses are pretty expensive. One organization is ITPS Canada:
https://itpscanada.com
A PPL is certainly a start, though a lot of experience on different types is required - but you have to start somewhere! Fly as many different types as you can as you learn and build experience!