test_qc_parallel: make a unique per-test working directory.
test_qc_parallel: make a unique per-test working directory.

This was reusing the same folder for every run, so test runs were not
isolated from each other.

This also now deletes the test directory once finished, which it didn't
before; perhaps this is undesirable?

neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox