Add silence_warnings for defining 'not_' prefix enum elements

To suppress the following warnings in tests.

```
~/rails/activerecord/lib/active_record/scoping/named.rb:190: warning: method redefined; discarding old not_sent
~/rails/activerecord/lib/active_record/scoping/named.rb:190: warning: previous definition of not_sent was here
```

activerecord/test/cases/enum_test.rb

@@ -580,7 +580,9 @@ def self.name; "Book"; end
       def self.name
         "Book"
       end
-      enum status: [:sent, :not_sent]
+      silence_warnings do
+        enum status: [:sent, :not_sent]
+      end
     end
 
     assert_match(expected_message, logger.logged(:warn).first)