These tests were always passing because they were stubbing the response
of the same method they were testing. For example, we were testing the
result of `Comment.public_for_api` and stubbing it at the same time.
So we're now stubbing the result of the associations; for example, in
order to test `Comment.public_for_api`, we're stubbing the response of
`Debate.public_for_api`. Now the tests fail if, for instance, the
implementation of `Comment.public_for_api` returns all comments.
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