Improve posts-sets testing

Add tests for failure states
Check the id of the returned post
Fix set being returned instead of post

Rearrange Controller_API_Sets_Posts::action_get_index() for readability.
Fix Controller_API_Sets_Posts::action_delete_index() - it was try to
remove the set from the set, instead of the post from the set. Also it
needed to return the post data,
not the set data.