TEST: added test for sct_otsu
TEST: added test for sct_otsu

Former-commit-id: a24c637b289cc62da01e499e75810549c9848256
Former-commit-id: 7fde57b52ee3682547b66a9ad73d14d8d121eaca
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox