Use "cacheable response" shared example in more places (#29419)

spec/controllers/custom_css_controller_spec.rb

@@ -14,17 +14,6 @@ describe CustomCssController do
       expect(response).to have_http_status(200)
     end
 
-    it 'returns public cache control header' do
-      expect(response.headers['Cache-Control']).to include('public')
-    end
-
-    it 'does not set cookies' do
-      expect(response.cookies).to be_empty
-      expect(response.headers['Set-Cookies']).to be_nil
-    end
-
-    it 'does not set sessions' do
-      expect(session).to be_empty
-    end
+    it_behaves_like 'cacheable response'
   end
 end