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heptapod-ci: let the number of CPU to be picked automatically

We now have a larger, bigger machine to run the tests. Let's let run-tests.py
pick the number of concurrent jobs.

--HG--
branch : stable
2 jobs for topic/stable/gitlab-ci-docker in 8 minutes and 22 seconds (queued for 6 minutes and 3 seconds)
Status Name Job ID Coverage
  Test
passed flake8 #48081

00:00:40

passed tests #48082

00:07:42