Updated posts exporter comment tests

fixes https://github.com/TryGhost/Team/issues/3505

I've investigated this, but luckily this is expected behaviour, and was just an error previously in the test:
- The `feedback_more_like_this` and `feedback_less_like_this` columns are only hidden if there is no newsletter with feedback enabled, or if member features are disabled.
- Having a post which is a draft, should not change the visible columns, only global settings should change the comments (so the format is consistent).
- Instead of that, the columns should be null, which means it will be visible but empty.

ghost/posts-service/test/PostsExporter.test.js

@@ -179,8 +179,7 @@ describe('PostsExporter', function () {
             post.status = 'draft';
             const posts = await exporter.export({});
 
-            // @TODO: why are these all nulls and not undefined?
-            // No feedback columns
+            // Feedback columns are empty, but present because of global settings (newsletter with feedback enabled)
             assert.equal(posts[0].feedback_more_like_this, null);
             assert.equal(posts[0].feedback_less_like_this, null);