Allow for disabling the default backend in tests at the command line

[matthewfeickert]

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        choices=["numpy", "tensorflow", "pytorch", "jax", "minuit"],

Is there a reason to not include numpy?

Originally posted by @matthewfeickert in https://github.com/scikit-hep/pyhf/pull/2340#discussion_r1346826533

Even though Issue https://github.com/scikit-hep/pyhf/issues/1454 was originally broached in the context of optional backends it would be nice to allow for diabling the default backend (numpy) if desired.