I'm not from the field, but I'd probably go for “sujets d'expérience”, preferably using one of “sujets d’une expérience” or “sujets de l'expérience” forms. Notice that just like the English term, it doesn't refer specifically to humans.
Of course it's hard to find scientific articles written in French, but there's a bunch of them (Google Scholar) that use this expression.
There are some cases where cobaye clearly don't fit, like the quite famous expérience de Milgram, where human aspects are studied. In Wikipedia's article they are referred to as “sujets [d’]expérience”, or simply by “sujets”.
L'université Yale à New Haven faisait paraître des annonces dans un journal local pour recruter les sujets d'une expérience sur l'apprentissage.
It's also the title that was chosen by a CNRS unit for a page where volunteers can enlist.