Make milestone_finder_spec faster

spec/lib/gitlab/github_import/milestone_finder_spec.rb

@@ -3,8 +3,8 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::GithubImport::MilestoneFinder, :clean_gitlab_redis_cache do
-  let!(:project) { create(:project) }
-  let!(:milestone) { create(:milestone, project: project) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:milestone) { create(:milestone, project: project) }
   let(:finder) { described_class.new(project) }
 
   describe '#id_for' do