list_render test: Improve filter coverage.
list_render test: Improve filter coverage.

This covers how we wire up the filter, and it
covers using `filterer` instead of `predicate`.

I also go away from the strange length-based
predicate that I had in the original test.
The intention behind the original test was
to show that filters could be more than simple
string-matching, but that was just a strange
way to demonstrate it.

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