component/nxdtest: v↑

This teaches nxdtest to detect leaked processes and to clean them up automatically instead of letting tests stuck because of them. See https://lab.nexedi.com/nexedi/nxdtest/commit/0ad45a9c and https://lab.nexedi.com/nexedi/nxdtest/merge_requests/9 for details.

New dependencies of nxdtest are:

• psutil (already covered by stack/slapos.cfg),
• pytest-timeout, and
• setproctitle

/cc @jerome