Make tests always use tempdir in Jenkins workspace to make sure the user_xattr option is set

user_xattr is required on the temporary folders created by the tests were Nuxeo Drive is working.

On the docker slaves the Jenkins workspace is a direct mount of the host so the user_xattr is set on the partition but we don't have control over /tmp.
As tempfile doesn't take into account the TMPDIR / TEMP / TMP environment variables we need to specify the tmp parent directory in each call to tempfile.mkdtemp, NamedTemporaryFile, ...