Their testable prediction is "no matter how the Public Good game looks exactly, it’s always the case that if endowments are too unequal, you’ll never get cooper

Their testable prediction is "no matter how the Public Good game looks exactly, it’s always the case that if endowments are too unequal, you’ll never get cooperation", quoted from the article. They themselves tested that prediction with experiments, but other are of course free to replicate them.