unit_testing: test_image: make tests more stringent
unit_testing: test_image: make tests more stringent


Former-commit-id: 8c0ef74fadc9bb552333e746769c0b0d340cfc14
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox