Exclude migration fixture from rubocop

.rubocop.yml

@@ -18,6 +18,7 @@ AllCops:
     - 'actionmailbox/test/dummy/**/*'
     - 'activestorage/test/dummy/**/*'
     - 'actiontext/test/dummy/**/*'
+    - 'railties/test/fixtures/db/**/*'
     - 'tools/rail_inspector/test/fixtures/*'
     - guides/source/debugging_rails_applications.md
     - guides/source/active_support_instrumentation.md