ENH: Test mne_make_scalp_surfaces

@dengemann see if you're okay with these changes. Should be ready for review/merge assuming Travis is happy. Requires updating of the testing dataset.
In other words, this check was meaningless so I removed it. Before it would check to see if `lh.seghead` existed, but if it did, even with `--overwrite` it wouldn't recalculate/overwrite it. What did you actually want this to do?