Fix mocks in background migrations specs

lib/gitlab/background_migration.rb

@@ -32,7 +32,6 @@ module Gitlab
     # arguments - The arguments to pass to the background migration's "perform"
     #             method.
     def self.perform(class_name, arguments)
-      puts class_name
       const_get(class_name).new.perform(*arguments)
     end
   end

spec/lib/gitlab/background_migration_spec.rb

@@ -10,12 +10,16 @@ describe Gitlab::BackgroundMigration do
 
   describe '.steal' do
     context 'when there are enqueued jobs present' do
-      let(:queue) { [double(:job, args: ['Foo', [10, 20]])] }
+      let(:job) { double(:job, args: ['Foo', [10, 20]]) }
+      let(:queue) { [job] }
 
       before do
         allow(Sidekiq::Queue).to receive(:new)
           .with(described_class.queue)
           .and_return(queue)
+
+        allow(job).to receive(:queue)
+          .and_return(described_class.queue)
       end
 
       it 'steals jobs from a queue' do