sct_testing: add --continue-from argument
sct_testing: add --continue-from argument


Former-commit-id: 95e842e66618809796d7f8f31a5d4e87cf7308a9
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox