Isolate stage_id reference clean up migration

This addreses a review remarks discussed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12785/diffs#note_35276344

spec/migrations/clean_stage_id_reference_migration_spec.rb

 require 'spec_helper'
 require Rails.root.join('db', 'migrate', '20170710083355_clean_stage_id_reference_migration.rb')
-require Rails.root.join('db', 'post_migrate', '20170628080858_migrate_stage_id_reference_in_background')
 describe CleanStageIdReferenceMigration, :migration, :sidekiq, :redis do
-  let(:migration) { MigrateStageIdReferenceInBackground::MIGRATION }
+  let(:migration) { 'MigrateBuildStageIdReference' }
   context 'when there are pending background migrations' do
     it 'processes pending jobs synchronously' do
       Sidekiq::Testing.disable! do
-        BackgroundMigrationWorker.perform_in(2.minutes, migration, [1])
-        BackgroundMigrationWorker.perform_async(migration, [1])
-        expect(Gitlab::BackgroundMigration).to receive(:perform).twice
+        BackgroundMigrationWorker.perform_in(2.minutes, migration, [1, 1])
+        BackgroundMigrationWorker.perform_async(migration, [1, 1])
+        expect(Gitlab::BackgroundMigration)
+          .to receive(:perform).twice.and_call_original
         migrate!
       end