Add back pytorch backend tests

[alexander-held]

Similar to #469: #547 disables the pytorch backend tests due to scikit-hep/pyhf#2554. Add back this backend test once supported via pyhf again.

[alexander-held]

The next pyhf release after scikit-hep/pyhf#2611 will remove this backend so at that point support from cabinetry can also be dropped.