[bug 617018] Don't show restricted forums

This tweaks the advanced search form code so that it doesn't let people
who aren't authorized to see forums search those forums or even know
of their existence.

It also does some mild cleanup in the tests and moves the
_restricted_forum maker to forum/tests/__init__.py .
Add tests for restricted forum searching

These tests cover some of the forum= querystring handling behavior to
fix bug #617018.