Tag migration specs with :sidekiq to clear queues between tests

This was failing in https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/55004375

spec/migrations/migrate_stages_statuses_spec.rb

 require 'spec_helper'
 require Rails.root.join('db', 'post_migrate', '20170711145558_migrate_stages_statuses.rb')
 
-describe MigrateStagesStatuses, :migration do
+describe MigrateStagesStatuses, :sidekiq, :migration do
   let(:jobs) { table(:ci_builds) }
   let(:stages) { table(:ci_stages) }
   let(:pipelines) { table(:ci_pipelines) }

spec/migrations/schedule_build_stage_migration_spec.rb

 require 'spec_helper'
 require Rails.root.join('db', 'post_migrate', '20180212101928_schedule_build_stage_migration')
 
-describe ScheduleBuildStageMigration, :migration do
+describe ScheduleBuildStageMigration, :sidekiq, :migration do
   let(:projects) { table(:projects) }
   let(:pipelines) { table(:ci_pipelines) }
   let(:stages) { table(:ci_stages) }