Right...I now realise that I misremembered. What I did was use a .dir-locals.el file in the root of a project to force the type. Contents:

((nil . ((projectile-project-type . lein-test))))

aha, nice

The more I think about it, though, the more I am thinking maybe if this is what I want I don't need Projectile and should just use, e.g., counsel-git.