動いた

application.html.erb_spec.rb

require 'spec_helper'

describe 'layouts/application' do
  describe 'rendered application layout' do
    subject { rendered }
    context 'when body_class is setted ' do
      before do
        view.content_for(:body_class, 'test_class')
        render
      end
      it 'should have body.test_class' do
        should have_selector('body.test_class', :count => 1)
      end
    end  
  end
end