tests: refactor z-scoring tests

[janfb]

the z-scoring tests need some refactoring:

https://github.com/sbi-dev/sbi/blob/dac03dbdcda3760e3de6d843d0ea4e4b644d8dc0/tests/sbiutils_test.py#L415

• 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