Cleanup belongs to tests

simplified logic to calculate number of queries by using assert_queries

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -391,8 +391,7 @@ def test_polymorphic_setting_foreign_key_after_nil_target_loaded
 
   def test_dont_find_target_when_foreign_key_is_null
     tagging = taggings(:thinking_general)
-    queries = assert_sql { tagging.super_tag }
-    assert_equal 0, queries.length
+    assert_queries(0) { tagging.super_tag }
   end
 
   def test_field_name_same_as_foreign_key