tests: introduce doctest_mock_ugettext to allow doctests of localized code

Future doctests will require some extra mocking, as the code-under-test uses
translation (ugettext aka '_') and its provider TurboGears2 needs a context.

Avoid this complexity by mocking ugettext as the identity function. This is
done by providing a pytest fixture 'doctest_mock_ugettext' that will mock
uggettext in the module that uses the fixture.