Suppress RuboCop's offense

Follow rubocop/rubocop#8707

This PR suppresses the following RuboCop's offense.

```console
% bundle exec rake
(snip)

Offenses:

spec/rubocop/cop/active_record_helper_spec.rb:6:3: W:
Lint/ConstantDefinitionInBlock: Do not define constants within a block.
  module RuboCop ...
    ^^^^^^^^^^^^^^

185 files inspected, 1 offense detected, 1 offense auto-correctable
RuboCop failed!
```

spec/rubocop/cop/active_record_helper_spec.rb

@@ -3,16 +3,8 @@
 RSpec.describe RuboCop::Cop::ActiveRecordHelper, :isolated_environment do
   include FileHelper
 
-  module RuboCop
-    module Cop
-      class Example < Cop
-        include ActiveRecordHelper
-      end
-    end
-  end
-
   let(:cop) do
-    RuboCop::Cop::Example.new
+    Class.new.extend RuboCop::Cop::ActiveRecordHelper
   end
 
   let(:schema_path) { 'db/schema.rb' }