Another useful test to add would be a test that checks round-tripping on all the functions.  That is, something like
```
randomz = np.abs(randn(1))[0] 

for funcnm in dir(astropy.cosmology.funcs):
    if funcnm.startswith('_'):
        continue
    func = getattr(astro.cosmology.funcs, funcnm)
    funcval = func(randomz)
    np.allclose(randomz, z_at_value(func, funcz, cosmo=cosmo))
```