CI: little scripts improvements

  -- in python: detection of a build failure, the tab should close nicely
  -- L2-nFAPi simulator: adding the num-ue-thread option
  -- Moved the VM destroy stage into the "VM-based test" branch.
  -- On "slave" job pipeline, moved the XML parsing after the git merge step so new test-cases are nicely taken care of.

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>