TEST: refactored test of sct_straighten_spinalcord, related to issue #506
TEST: refactored test of sct_straighten_spinalcord, related to issue #506

Former-commit-id: f531b6c0b4394bf20d37fdbcf801f49a4a12b1f7 [formerly e3bd27ae113350e1fed8126bbc0d06166662dd93]
Former-commit-id: 6922199c6f7e8baa554453d6bd59f2b95006d72f
Former-commit-id: 6e3e876c7374df2117cd58a290b5220de583290c
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox