Add hook_for :fixture_replacement in scaffold generator for functional test template.

When I run `rails g scaffold post` with factore girl, the controller test file ` test/controllers/posts_controller_test.rb` likes this:

```ruby
require 'test_helper'

class PostsControllerTest < ActionController::TestCase
  setup do
    @post = posts(:one)
  end

  ...

end
```
It write in the fixture style but not factory girl style, so the functional test can not pass.