Add specs for stage_id reference cleanup migration

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')
 
-describe CleanStageIdReferenceMigration, :migration, :sidekiq do
-  context 'when there are enqueued background migrations' do
-    pending 'processes enqueued jobs synchronously' do
-      fail
-    end
-  end
+describe CleanStageIdReferenceMigration, :migration, :sidekiq, :redis do
+  let(:migration) { MigrateStageIdReferenceInBackground::MIGRATION }
+
+  context 'when there are pending background migrations' do
+    it 'processes enqueued jobs synchronously' do
+      Sidekiq::Testing.disable! do
+        BackgroundMigrationWorker.perform_in(2.minutes, migration, [1])
+        BackgroundMigrationWorker.perform_async(migration, [1])
 
-  context 'when there are scheduled background migrations' do
-    pending 'immediately processes scheduled jobs' do
-      fail
+        expect(Gitlab::BackgroundMigration).to receive(:perform).twice
+
+        migrate!
+      end
     end
   end
 
   context 'when there are no background migrations pending' do
-    pending 'does nothing' do
-      fail
+    it 'does nothing' do
+      Sidekiq::Testing.disable! do
+        expect(Gitlab::BackgroundMigration).not_to receive(:perform)
+
+        migrate!
+      end
     end
   end
 end