Address inconsistency between unit tests of NFW orbits with vs. without dynamical friction

[aphearin]

The unit tests of the NFW orbits written by @ngluck were originally developed to be identical to each other, except only for whether the kernel includes/excludes the impact of dynamical friction. But the DF case does not use halotools.empirical_models.NFWPhaseSpace, whereas the no-DF case does, so there must be some significant difference between these two unit-testing modules. This difference could be innocuous, in which case this issue can just be closed, but we should double-check that the unit tests of these two kernels are being carried out as expected.