[views] fix possible infinite recursion loop in rssitem view

I found this while executing some AutomaticWebTest on dependent cubes, but this case can occur anyway.

rssitem call rssitem view recursively following "comments" relation, so we may
enter in a loop here.

Fix this by using a "seen" set() which is passed resursively and we skip
already processed entities.
Add a basic test using a comment commenting a comment commenting the first one.