Parametrize `test_classifications`

Refs #234

It seems slower with parametrize (time tox -- tests/test_elements_worker/test_classifications.py )

With parametrize:

real	0m26,853s
user	0m9,453s
sys	0m0,800s

Without:

real	0m15,225s
user	0m5,799s
sys	0m0,414s