Add :sidekiq group to cleanup redis in after callback

spec/lib/gitlab/background_migration/delete_diff_files_spec.rb

 require 'spec_helper'
 
-describe Gitlab::BackgroundMigration::DeleteDiffFiles, :migration, schema: 20180619121030 do
+describe Gitlab::BackgroundMigration::DeleteDiffFiles, :migration, :sidekiq, schema: 20180619121030 do
   describe '#perform' do
     context 'when diff files can be deleted' do
       let(:merge_request) { create(:merge_request, :merged) }

spec/lib/gitlab/background_migration/schedule_diff_files_deletion_spec.rb

 require 'spec_helper'
 
-describe Gitlab::BackgroundMigration::ScheduleDiffFilesDeletion, :migration, schema: 20180619121030 do
+describe Gitlab::BackgroundMigration::ScheduleDiffFilesDeletion, :migration, :sidekiq, schema: 20180619121030 do
   describe '#perform' do
     let(:merge_request_diffs) { table(:merge_request_diffs) }
     let(:merge_requests) { table(:merge_requests) }