I can only second Pilot DAR's advice. The Flight Test Community is quite small - a running joke is that "If we haven't got a few mutual friends, someone's lying about their resume". In any company, even large ones, the number of people engaged in Flight Testing (of all levels & specialties right down to the essential Mechanics that keep everything flying) is very small. So my advice is join the Flight Test Department in any role available to you & then be prepared to take advantage of any airborne testing offered. There may be plenty of Graveyard shifts in your future (any Simulator time will be between midnight & 4 am) but you will eventually get to go test.
Having been involved with Avionics Flight Testing, I should point out that one of the largest chunks of Flight Testing in any program is the Autopilot - even more if clearance to lower minima like CAT II or CAT III is required.
Last edited by ICT_SLB; 17th August 2021 at 21:51.