TST: use numpy.testing.overrides API for NEP 18/35 completion tests

[neutrinoceros]

I'm working on expanding the definition of array_functions as retrieve by the numpy.testing API, to include NEP 35 functions
numpy/numpy#27490

I anticipate this will break completion tests in unyt. Implementing NEP 35 is a large task (#518) that I reserve for the next feature release, but I'd like to make unyt's test suite future-proof before that, by explicitely listing those functions as not implemented in uny 3.0.4

[neutrinoceros]

turns out we're not using numpy.testing.overrides.get_overridable_numpy_array_functions yet, so this is actually a non issue for unyt 3.0.4.
Nonetheless, we should be using this API for our completion tests, so I'll re-purpose this issue for unyt 3.1.0