test_backup_restore: do not fail on missing KrbLastSuccessfulAuth
Since FreeIPA 4.5.1 now sets 'Disable last successful auth' option by
default (see https://pagure.io/freeipa/issue/5313), the
'KrbLastSuccessfulAuth' may not always be present on the user entry. The
restored entry checker in backup/restore suite should consider this.
https://pagure.io/freeipa/issue/6956
Reviewed-By: Martin Basti <mbasti@redhat.com>