sct_testing: add --check-filesystem feature
sct_testing: add --check-filesystem feature

Changes:

- Add --check-filesystem feature, which makes the test sequence
violently crash in case tests modified somewhere they shouldn't have.
- Fixup path lookup mistakes in test_integrity()

Example locations:

- clean # path_testing (ie. --execution-folder or tempfile)
- clean/sct_detect_pmj # per-test execution folder (using relative
path_output for sct_testing.main())
- clean/sct_detect_pmj/sct_detect_pmj.log # logfile
- clean/sct_detect_pmj/sct_detect_pmj/ # -ofolder or -o (again, using
relative path_output)


Former-commit-id: 5d9bc216018adb51f760fc9e6765a58bcc4d4c8f
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox