Add back TensorFlow backend tests

[alexander-held]

#468 removed TensorFlow temporarily from the tests using all available pyhf backends. This is due to tensorflow / tensorflow-probability diverging and addressed in scikit-hep/pyhf#2452. Add back this backend to the tests once the next pyhf version is available.

[matthewfeickert]

As a note to myself, once I release pyhf v0.7.7 with the patches for this in it, I need to report back here.

[alexander-held]

Following scikit-hep/pyhf#2609 this backend is removed from pyhf so it can be completely removed from cabinetry testing as well in future releases.