Add test helper that returns content item

stub_conditional_loader_returns_content_item helper corresponds to GdsApi
adapters helpers stubs:
- https://github.com/alphagov/gds-api-adapters/blob/4d4b2ab6969f333eb1cab94ff8e93d7ff8aeebf1/lib/gds_api/test_helpers/content_store.rb#L21
- https://github.com/alphagov/gds-api-adapters/blob/4d4b2ab6969f333eb1cab94ff8e93d7ff8aeebf1/lib/gds_api/test_helpers/publishing_api.rb#L210

The Struct mimics WebMock response returned by the stubs.  The `to_h` method
parses JSON, like WebMock middleware would.

I considered returning a RestClient::Response but has a lot of unnecessary
complexity (e.g. redirect history). A lightweight Response struct seems
sufficient.

Author: AgaDufrat

lib/govuk_content_item_loader.rb

@@ -1,3 +1,4 @@
 require "govuk_content_item_loader/govuk_graphql_traffic_rates"
 require "govuk_content_item_loader/govuk_conditional_content_item_loader"
 require "govuk_content_item_loader/version"
+require "govuk_content_item_loader/test_helpers"

lib/govuk_content_item_loader/test_helpers.rb

@@ -0,0 +1,34 @@
+require "json"
+
+module GovukConditionalContentItemLoaderTestHelpers
+  Response = Struct.new(:status, :body, :headers) do
+    def to_h
+      JSON.parse(body)
+    end
+  end
+
+  # Stubs the loader returns a content item
+  # The following options can be passed in:
+  #
+  #   :max_age  will set the max-age of the Cache-Control header in the response. Defaults to 900
+  #   :private  if true, the Cache-Control header will include the "private" directive. By default it
+  #             will include "public"
+  def stub_conditional_loader_returns_content_item(body, options = {})
+    body.to_json unless body.is_a?(String)
+    max_age = options.fetch(:max_age, 900)
+    visibility = options[:private] ? "private" : "public"
+
+    response = Response.new(200, body, {
+      cache_control: "#{visibility}, max-age=#{max_age}",
+      date: Time.now.httpdate,
+    })
+
+    loader = instance_double(GovukConditionalContentItemLoader, load: response)
+
+    allow(GovukConditionalContentItemLoader).to receive(:new).with(request: anything)
+      .and_return(loader)
+    allow(loader).to receive(:load).and_return(body)
+
+    response
+  end
+end