tests: use test_context for tests needing internationalization (bis)

Commit 8e3137064ab6 already introduced the use of test_context to cover
internationalization in the test suite, instead of setting it up globally.

When making changes related to formencode internationalization, a new batch
of internationalization errors popped up and a commit was made to fix them.
However, after some later refactoring, it looked as if the commit was not
needed anymore.

In Turbogears context, it was indeed not necessary as long as we still had
some places that used the dummy formencode.api._ rather than a real version
of _ (ugettext).
After cleaning up that forgotten import, the test internationalization
errors popped up again. Hence, we need to reapply the earlier commit (with
some changes).