BUG: improved sensitivity of sct_propseg test. Fixed #792
BUG: improved sensitivity of sct_propseg test. Fixed #792

Former-commit-id: 9b9a039007a3123c6d78e466d36c711355175e21
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox