test_caless: adjust try/except to capture also IOError
While testing on RHEL we are getting IOError instead of OSError.
Add also IOError to except clause.
This is mostly for compatibility reasons however should not cause
any issue as IOError is alias for OSError on Python3.
https://pagure.io/freeipa/issue/7439
Reviewed-By: Christian Heimes <cheimes@redhat.com>