Add pending set of specs for stage_id 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
+
+  context 'when there are scheduled background migrations' do
+    pending 'immediately processes scheduled jobs' do
+      fail
+    end
+  end
+
+  context 'when there are no background migrations pending' do
+    pending 'does nothing' do
+      fail
+    end
+  end
+end