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)

test/test_helpers/controller_helper.rb

@@ -0,0 +1,47 @@
+module ControllerHelper
+  # A custom Shoulda matcher that tests that a controller's index endpoint
+  # responds to a search correctly. See https://thoughtbot.com/blog/shoulda-matchers.
+  #
+  # Usage:
+  #
+  #   # Tests that `/tags.json?search[name]=touhou` returns the `touhou` tag.
+  #   subject { TagsController }
+  #   setup { @touhou = create(:tag, name: "touhou") }
+  #   should respond_to_search(name: "touhou").with { @touhou }
+  #
+  def respond_to_search(search_params)
+    RespondToSearchMatcher.new(search_params)
+  end
+
+  class RespondToSearchMatcher < Struct.new(:params)
+    def description
+      "should respond to a search for #{params}"
+    end
+
+    def matches?(subject, &block)
+      search_params = { search: params }
+      expected_items = @test_case.instance_eval(&@expected)
+
+      @test_case.instance_eval do
+        # calls e.g. "wiki_pages_path" if we're in WikiPagesControllerTest.
+        index_url = send("#{subject.controller_path}_path")
+        get index_url, as: :json, params: search_params
+
+        expected_ids = Array(expected_items).map(&:id)
+        responded_ids = response.parsed_body.map { |item| item["id"] }
+
+        assert_response :success
+        assert_equal(expected_ids, responded_ids)
+      end
+    end
+
+    def with(&block)
+      @expected = block
+      self
+    end
+
+    def in_context(test_case)
+      @test_case = test_case
+    end
+  end
+end