Fix background migration cleanup specs

We need to use a spy because an `after` RSpec hook is also going to call
the migration we want to test, so we need to use `have_received`
expectation. See gitlab-org/gitlab-ce#35351 for more details.

spec/migrations/clean_stage_id_reference_migration_spec.rb

@@ -2,28 +2,32 @@ require 'spec_helper'
 require Rails.root.join('db', 'migrate', '20170710083355_clean_stage_id_reference_migration.rb')
 
 describe CleanStageIdReferenceMigration, :migration, :sidekiq, :redis do
-  let(:migration) { 'MigrateBuildStageIdReference' }
+  let(:migration_class) { 'MigrateBuildStageIdReference' }
+  let(:migration) { spy('migration') }
+
+  before do
+    allow(Gitlab::BackgroundMigration.const_get(migration_class))
+      .to receive(:new).and_return(migration)
+  end
 
   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, 1])
-        BackgroundMigrationWorker.perform_async(migration, [1, 1])
-
-        expect(Gitlab::BackgroundMigration)
-          .to receive(:perform).twice.and_call_original
+        BackgroundMigrationWorker.perform_in(2.minutes, migration_class, [1, 1])
+        BackgroundMigrationWorker.perform_async(migration_class, [1, 1])
 
         migrate!
+
+        expect(migration).to have_received(:perform).with(1, 1).twice
       end
     end
   end
-
   context 'when there are no background migrations pending' do
     it 'does nothing' do
       Sidekiq::Testing.disable! do
-        expect(Gitlab::BackgroundMigration).not_to receive(:perform)
-
         migrate!
+
+        expect(migration).not_to have_received(:perform)
       end
     end
   end