Implement `json_response` as a `let` variable

This is not a good idea to memoize `json_response` using an instance
variable because `rspec-retry` doesn't clear instance variables on
retries, only `let` variables.

This will avoid issues where retries would fail on a different line that
the original failure, blurrying what's the real failure.

Also, automatically add api: true to specs under
/spec/requests/(ci/)?api/, and include JsonHelpers in controller, request
and API specs.
Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/requests/api/projects_spec.rb

 # -*- coding: utf-8 -*-
 require 'spec_helper'
 
-describe API::Projects, api: true  do
-  include ApiHelpers
+describe API::Projects, :api  do
   include Gitlab::CurrentSettings
+
   let(:user) { create(:user) }
   let(:user2) { create(:user) }
   let(:user3) { create(:user) }

spec/spec_helper.rb

@@ -43,8 +43,14 @@ RSpec.configure do |config|
   config.include ActiveSupport::Testing::TimeHelpers
   config.include StubGitlabCalls
   config.include StubGitlabData
+  config.include ApiHelpers, :api
 
   config.infer_spec_type_from_file_location!
+
+  config.define_derived_metadata(file_path: %r{/spec/requests/(ci/)?api/}) do |metadata|
+    metadata[:api] = true
+  end
+
   config.raise_errors_for_deprecations!
 
   config.before(:suite) do

spec/support/api_helpers.rb

@@ -49,8 +49,4 @@ module ApiHelpers
         ''
       end
   end
-
-  def json_response
-    @_json_response ||= JSON.parse(response.body)
-  end
 end

spec/support/json_response_helpers.rb

+shared_context 'JSON response' do
+  let(:json_response) { JSON.parse(response.body) }
+end
+
+RSpec.configure do |config|
+  config.include_context 'JSON response', type: :controller
+  config.include_context 'JSON response', type: :request
+  config.include_context 'JSON response', :api
+end