Remove AppTester.setup.kv(fn)

Once #121 lands, we no longer have use for `AppTester.setup.kv(fn)`, since we now set properties directly on api.kv.store (instead of us holding onto a temporary object before putting it on the api). If people need to setup their kv store using a function, they can now just use `AppTester.setup(fn)`, since it gives them the api as an arg.