fix intermittent test failure

should fix the intermittent errors of this form

```
======================================================================
FAIL: testBasicRebuild (casexml.apps.case.tests.test_rebuild.CaseRebuildTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "corehq/ex-submodules/casexml/apps/case/tests/test_rebuild.py", line 92, in testBasicRebuild
    self.assertEqual(case.p2, 'p2-1') # updated (back!)
AssertionError: u'p2-2' != 'p2-1'
```