maintenance: fix post pruning test.

Test mistakenly passed because we used `assert(a, b)` instead of
`assert_equal(a, b)`.

app/logical/danbooru_maintenance.rb

@@ -6,7 +6,7 @@ module DanbooruMaintenance
   end
 
   def daily
-    safely { PostPruner.new.prune! }
+    safely { PostPruner.prune! }
     safely { Delayed::Job.where('created_at < ?', 45.days.ago).delete_all }
     safely { PostDisapproval.prune! }
     safely { regenerate_post_counts! }