tests: enable import checker for tests/**.py files
Several known-bad files are excluded as they couldn't be trivially fixed.
In principle, we should fix them first, however, it would have more risk
to keep Py3k porting going without the test coverage.
Still contrib/**.py aren't covered, which needs another round.
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors