sct_testing: set path_output only if not set already
sct_testing: set path_output only if not set already


Former-commit-id: eb3c36bf8036a3e98bda0d6e2eef8291373b3a82
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox