Playtest Games
I participated in a now-defunct game here on RPoL a while back that was a test of the Fate Core game creation rules. The GM that ran that test was up-front about it (always a good thing), and had a lot of players with a high degree of interest in actually playing in a game they helped design, but the project foundered for one or another reason, and the GM wrapped it up, floated a proposal for a different Fate game, and moved on.
It happens. It takes a rare breed of player to be up for a playtest, especially of a completely new system or milieu. Many players want to play more than they want to test, and this is not a bad thing, but a large number of players of this sort is not what you want for a playtest group.
Definitely be honest about your intentions. Communicate clearly what you are seeking to accomplish, and if you have design goals or milestones that won't compromise the play end of things, let your testers know.
I have some stuff I might eventually want to test here. I'm sure if I asked, I'd have some players interested in playing. Whether I'd see a lot of players interested in helping to test, well, you rolls the dice and you takes your chances.
Oh, be willing to accept what your playtesters tell you about their experiences in the game, especially about any criticisms or observations they may have.
I had one across-a-table playtest about a year ago that taught me a lot about game design. I've been chewing on what I learned in that exercise for a while now, and think I have mapped a path forward that will greatly improve my finished product.
(A lot happened in that intervening year to tear me away from my group, or I would have tried to run a second test by now. Perhaps, even though it was frustrating in some ways, this was a good thing. I may have developed some patience I did not have previously.)