unit_testing: image: add cropping test
unit_testing: image: add cropping test


Former-commit-id: 9b952baa109d9826184f4300b32bf756f4b0a342
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox