Make the "rst" converter tests independent of the local Docutils configuration.

[gmilde]

Running pytest on a recent git checkout fails due to my local Docutils configuration differing from default settings.

I suppose the reason is, that I activate the smart_quotes transform in my /etc/docutils.conf. As a result, ASCII-Quotes are converted to typographcal quotes, e.g.:

...
src/moin/converters/_tests/test_rst_in_out.py:202: 
[...]
>       assert self.handle_output(out) == output
E       assert '\n.. figure:...o long ago.\n' == '\n.. figure:...o long ago.\n'
E         Skipping 106 identical leading characters in diff, use -v to show
E         - laced the "MoinMoin Man"
E         ?           ^            ^
E         + laced the “MoinMoin Man”
E         ?           ^            ^
E              logo long ago.
...

In Docutils, the test-publisher is always started with the internal "_disable_config" setting, e.g.

        settings_overrides = {
            '_disable_config': True,
            # other settings,
            }
        result = core.publish_string(
            input_sample, writer=html5_polyglot.Writer(),
            settings_overrides=settings_overrides))

But I did not find where in the moin test suite the Docutils Publisher is called.