TEST: added test for sct_get_centerline_from_labels
TEST: added test for sct_get_centerline_from_labels

Former-commit-id: 4dee863000f3d5f185fc7d2bd6a8e86122ee504f
Former-commit-id: e88fd10ddced098c7b227c14b06593e2eab4d48e
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox