This teaches nxdtest to detect leaked processes and to clean them up
automatically instead of letting tests stuck because of them. See
nxdtest@0ad45a9c and
nxdtest!9 for details.

New dependencies of nxdtest are:

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

/reviewed-by @jerome
/reviewed-on !1035