Fix unit tests

Author: Vasileios Karakasis

reframe/core/schedulers/pbs.py

@@ -20,7 +20,7 @@
 import reframe.utility.osext as osext
 from reframe.core.backends import register_scheduler
 from reframe.core.config import settings
-from reframe.core.exceptions import JobSchedulerError
+from reframe.core.exceptions import (JobError, JobSchedulerError)
 from reframe.utility import seconds_to_hms