lib/lp/services/database/doc/textsearching.txt fails rarely/intermittently in parallel tests

[Francesco Banconi]

This is the test list for worker 0: http://pastebin.ubuntu.com/979703/

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FAILURE: lib/lp/services/database/doc/textsearching.txt
worker ID: worker-0
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Traceback (most recent call last):
  File "/usr/lib/python2.6/unittest.py", line 279, in run
    testMethod()
  File "/usr/lib/python2.6/doctest.py", line 2152, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for textsearching.txt
  File "lib/lp/services/database/doc/textsearching.txt", line 0

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File "lib/lp/services/database/doc/textsearching.txt", line 579, in textsearching.txt
Failed example:
    nl_term_candidates('firefox foo-bar give me trouble')
Differences (ndiff with -expected +actual):
    - [u'firefox', u'foo', u'bar', u'foobar', u'give', u'troubl']
    ? - ^ --- ^^^
    + [u'ive', u'bl', u'bar', u'foobar', u'give', u'troubl']
    ? ^ ^^