Not all the functions in `funcs` are suitable to be tested here (like `angular_diameter_distance_z1z2` and `z_at_value` itself). Any new functions added also may not be suitable, and `dir` would result in unexpected failing tests. So it seems better to me to explicitly list all the ones we want, rather than excluding the unsuitable ones.

But if this argument doesn't persuade you, I'm happy to change it to use `dir` :)