testnode: Build NumPy/friends & Ruby gems in parallel
Following @rafael advice on a related commit to slaprunner
https://lab.nexedi.com/nexedi/slapos/commit/c8c81baa https://lab.nexedi.com/nexedi/slapos/merge_requests/22#note_1780
let's also leverage multicore when building NumPy- & Ruby- related software under testnodes.
/cc @seb