Test if job can be scheduled before adding to backfill test

Add a call to acct_policy_job_runnable() to check if a job should have been marked with
AssociationResourceLimit.  This avoids trying to backfill any job who wouldn't be allowed
to run now anyway.
Patch bf_acct_policy_check_2.2.6.patch from Bjørn-Helge Mevik, University of Oslo