TEST: added test for sct_extract_metric
TEST: added test for sct_extract_metric

Former-commit-id: 0490788ebb0788c9b286eefa996c302055d6ab81 [formerly 4c05ab4f72477e4121402e29808b1ffbd7f5ccb9]
Former-commit-id: 9792ce49000999e096ca62c3b279795286803ee1
Former-commit-id: 868c7b911df1991afd98e49c7c156b43f8b8a8d8
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox