tests: add respond_to_search test helper.

Add a `respond_to_search` test helper for concisely testing that a
controller's index action correctly responds to a search. Usage:

    # Tests that `/tags.json?search[name]=touhou` returns the `touhou` tag.

    setup { @touhou = create(:tag, name: "touhou") }
    should respond_to_search(name: "touhou").with { @touhou }

test/test_helper.rb

@@ -61,6 +61,8 @@ class ActiveSupport::TestCase
 end
 
 class ActionDispatch::IntegrationTest
+  extend ControllerHelper
+
   register_encoder :xml, response_parser: ->(body) { Nokogiri.XML(body) }
 
   def method_authenticated(method_name, url, user, **options)