Functional tests sometimes randomly failing

[Jun Omae]

We recently encountered random failures of functional tests on Travis CI. Probably, Proxy.get_source() in better_twill.py has something wrong, e.g race condition.

I'd like to fix the issue before release of 1.6 because it leads stopping release procedure when release tagging.

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ERROR: runTest (trac.wiki.tests.functional.TestWikiDelete)
Delete a wiki page.
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Traceback (most recent call last):
  File "/home/travis/build/edgewall/trac/trac/wiki/tests/functional.py", line 51, in runTest
    tc.find("Are you sure you want to completely delete this page?")
  File "/home/travis/build/edgewall/trac/trac/tests/functional/better_twill.py", line 191, in find
    raise AssertionError("Regex didn't match: {!r} not found in {}"
AssertionError: Regex didn't match: 'Are you sure you want to completely delete this page?' not found in file:///home/travis/build/edgewall/trac/testenv/trac/log/TestWikiDelete.html

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ERROR: runTest (trac.ticket.tests.functional.admin.TestAdminMilestoneDetail)
Admin modify milestone details
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Traceback (most recent call last):
  File "/home/travis/build/edgewall/trac/trac/ticket/tests/functional/admin.py", line 298, in runTest
    self._tester.login('user')
  File "/home/travis/build/edgewall/trac/trac/tests/functional/tester.py", line 50, in login
    tc.find("Login")
  File "/home/travis/build/edgewall/trac/trac/tests/functional/better_twill.py", line 191, in find
    raise AssertionError("Regex didn't match: {!r} not found in {}"
AssertionError: Regex didn't match: 'Login' not found in file:///home/travis/build/edgewall/trac/testenv/trac/log/TestAdminMilestoneDetail.html