[MAINT, MRG] Better test for `mne.surface._voxel_neighbors` (#9950)
[MAINT, MRG] Better test for `mne.surface._voxel_neighbors` (#9950)

* better test for _voxel_neighbors

* bad import
Eric89GXL/mne-python,pravsripad/mne-python,wmvanvliet/mne-python,wmvanvliet/mne-python,drammock/mne-python,Eric89GXL/mne-python,Teekuningas/mne-python,mne-tools/mne-python,Teekuningas/mne-python,drammock/mne-python,larsoner/mne-python,larsoner/mne-python,kingjr/mne-python,olafhauk/mne-python,olafhauk/mne-python,wmvanvliet/mne-python,pravsripad/mne-python,Teekuningas/mne-python,mne-tools/mne-python,olafhauk/mne-python,pravsripad/mne-python,mne-tools/mne-python,larsoner/mne-python,drammock/mne-python,kingjr/mne-python,kingjr/mne-python