Skip broken symlink test on Windows

Python 3.2 introduced os.symlink() on Windows platforms, but it raises
NotImplementedError if the underlying OS is too old to support symlinks
(like what we have on winbot).

Fixes #13.

src/zope/testing/setupstack.txt

@@ -93,7 +93,7 @@ On Unix systems, broken symlinks can cause problems because the chmod
 attempt by the teardown hook will fail; let's set up a broken symlink as
 well, and verify the teardown doesn't break because of that:
 
-    >>> if hasattr(os, 'symlink'):
+    >>> if sys.platform != 'win32':
     ...     os.symlink('NotThere', 'BrokenLink')
 
 When tearDown is called: