Making the exception/exception test work:

  Always putting the process in the process_to_run list is bad.
  Indeed, when canceling tasks (e.g., with SIMIX_host_execution_cancel),
  the action will be cancelled and the corresponding process will be
  run again when the action is popped from the list. This leads to very
  surprising behavior since the action is only popped way after, generally
  after calling surf_solve.... This is why the fix has two parts:
  1) Do not put in the process_to_run list when canceling the action
  2) Once all process have been run, pop the surf actions in case some have been modified