stub config settings in spec

spec/mailers/notify_spec.rb

@@ -1096,12 +1096,9 @@ describe Notify do
   end
 
   describe 'email has correct subject' do
-    let(:gitlab_subject_suffix) { Gitlab.config.gitlab.email_subject_suffix }
-
     it 'has correct suffix' do
-      if gitlab_subject_suffix.length > 0
-        is_expected.to have_subject gitlab_subject_suffix
-      end
+      stub_config_setting(email_subject_suffix: 'A Nice Suffix')
+      is_expected.to have_subject /\| A Nice Suffix$/
     end
   end
 end