Fix `bin/rails db:setup` and `bin/rails db:test:prepare` create  wrong ar_internal_metadata's data for a test database.

  Before:
  ```
  $ RAILS_ENV=test rails dbconsole
  > SELECT * FROM ar_internal_metadata;
  key|value|created_at|updated_at
  environment|development|2017-09-11 23:14:10.815679|2017-09-11 23:14:10.815679
  ```

  After:
  ```
  $ RAILS_ENV=test rails dbconsole
  > SELECT * FROM ar_internal_metadata;
  key|value|created_at|updated_at
  environment|test|2017-09-11 23:14:10.815679|2017-09-11 23:14:10.815679
  ```

  Fixes #26731.