Minor test fixture fix

Fixed a FK relationship in a test fixture to prevent spurious
crashes when running bin/openquake --lhc.

The problem manifests itself like so:
<pre>
$ bin/openquake --lhc
calc_id | status | last_update | description
38 | pending | 2013-07-29 08:58:14  | Simple Fault Demo, Classical PSHA
39 | pending | 2013-07-29 08:58:23  | Simple Fault Demo, Classical PSHA
40 | failed | 2013-07-29 08:58:23  | Simple Fault Demo, Classical PSHA
41 | failed | 2013-07-29 08:58:23  | Simple Fault Demo, Classical PSHA
42 | failed | 2013-07-29 08:58:24  | Simple Fault Demo, Classical PSHA
43 | failed | 2013-07-29 08:58:24  | Simple Fault Demo, Classical PSHA
44 | failed | 2013-07-29 08:58:24  | Simple Fault Demo, Classical PSHA
46 | failed | 2013-07-29 08:58:26  | Simple Fault Demo, Classical PSHA
47 | failed | 2013-07-29 08:58:27  | Simple Fault Demo, Classical PSHA
50 | failed | 2013-07-29 08:58:37  | Disaggregation Hazard Demo
51 | failed | 2013-07-29 08:58:37  | Disaggregation Hazard Demo
52 | failed | 2013-07-29 08:58:51  | Probabilistic Event-Based Hazard Demo
54 | failed | 2013-07-29 08:58:59  | Probabilistic Event-Based Hazard Demo
55 | failed | 2013-07-29 08:58:59  | Probabilistic Event-Based Hazard Demo
56 | failed | 2013-07-29 08:58:59  | Probabilistic Event-Based Hazard Demo
57 | failed | 2013-07-29 08:59:00  | Probabilistic Event-Based Hazard Demo
58 | failed | 2013-07-29 08:59:00  | Probabilistic Event-Based Hazard Demo
89 | failed | 2013-07-29 08:59:06  | Scenario QA Test, Case 1
137 | successful | 2013-07-29 09:03:52  | Simple Fault Demo, Classical PSHA
138 | successful | 2013-07-29 09:03:52  | Simple Fault Demo, Classical PSHA
139 | successful | 2013-07-29 09:03:52  | Simple Fault Demo, Classical PSHA
Traceback (most recent call last):
  File "bin/openquake", line 506, in <module>
    main()
  File "bin/openquake", line 453, in main
    list_hazard_calculations()
  File "bin/openquake", line 293, in list_hazard_calculations
    _print_calcs_summary(hcs)
  File "bin/openquake", line 316, in _print_calcs_summary
    latest_job = calc.oqjob
  File "/Users/larsbutler/.virtualenvs/openquake/lib/python2.7/site-packages/django/db/models/fields/related.py", line 239, in __get__
    rel_obj = self.related.model._base_manager.using(db).get(**params)
  File "/Users/larsbutler/.virtualenvs/openquake/lib/python2.7/site-packages/django/db/models/query.py", line 349, in get
    % self.model._meta.object_name)
openquake.engine.db.models.DoesNotExist: OqJob matching query does not exist.
</pre>

I narrowed down the issue to this single test.