TEST: updated test for compute_tsnr (flag changed)
TEST: updated test for compute_tsnr (flag changed)

Former-commit-id: f405a47c99c6588ce009cf3fc7d2e554f999feb3
Former-commit-id: e972165aea2d12ea6adf4f3eb55aa07125786b3d
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox