TST: Add unit test for kwarg of np.einsum
TST: Add unit test for kwarg of np.einsum

Ensure that explicitly stating out=None does not raise an error in
np.einsum, see #15776 and #15256.

mattip/numpy,mattip/numpy,pbrod/numpy,jakirkham/numpy,madphysicist/numpy,grlee77/numpy,WarrenWeckesser/numpy,mhvk/numpy,pbrod/numpy,mhvk/numpy,pbrod/numpy,seberg/numpy,seberg/numpy,pdebuyl/numpy,abalkin/numpy,charris/numpy,endolith/numpy,madphysicist/numpy,endolith/numpy,anntzer/numpy,jakirkham/numpy,anntzer/numpy,simongibbons/numpy,mattip/numpy,mhvk/numpy,rgommers/numpy,rgommers/numpy,WarrenWeckesser/numpy,pbrod/numpy,numpy/numpy,abalkin/numpy,endolith/numpy,pdebuyl/numpy,pdebuyl/numpy,rgommers/numpy,numpy/numpy,charris/numpy,madphysicist/numpy,pdebuyl/numpy,anntzer/numpy,grlee77/numpy,charris/numpy,seberg/numpy,jakirkham/numpy,simongibbons/numpy,mhvk/numpy,mhvk/numpy,numpy/numpy,jakirkham/numpy,rgommers/numpy,endolith/numpy,simongibbons/numpy,jakirkham/numpy,grlee77/numpy,abalkin/numpy,WarrenWeckesser/numpy,madphysicist/numpy,numpy/numpy,seberg/numpy,charris/numpy,madphysicist/numpy,anntzer/numpy,grlee77/numpy,simongibbons/numpy,pbrod/numpy,WarrenWeckesser/numpy,mattip/numpy,grlee77/numpy,simongibbons/numpy,WarrenWeckesser/numpy