tests: refactor z-scoring tests

[janfb]

the z-scoring tests need some refactoring:

sbi/tests/sbiutils_test.py

Line 415 in dac03db

def test_z_scoring_structured(z_x, z_theta, builder):

• variable names are confusing, e.g., net should be called build_fun
• there are many duplicate test cases
• why pass net(t_batch, t_batch) and not something like net(theta, x).
• add a case for mnle