Extract background migratons queue class method

lib/gitlab/background_migration.rb

 module Gitlab
   module BackgroundMigration
+    def self.queue
+      BackgroundMigrationWorker.sidekiq_options['queue']
+    end
+
     # Begins stealing jobs from the background migrations queue, blocking the
     # caller until all jobs have been completed.
     #
     # steal_class - The name of the class for which to steal jobs.
     def self.steal(steal_class)
-      queue = Sidekiq::Queue
-        .new(BackgroundMigrationWorker.sidekiq_options['queue'])
+      queue = Sidekiq::Queue.new(self.queue)
 
       queue.each do |job|
         migration_class, migration_args = job.args

spec/lib/gitlab/background_migration_spec.rb

 require 'spec_helper'
 
 describe Gitlab::BackgroundMigration do
+  describe '.queue' do
+    it 'returns background migration worker queue' do
+      expect(described_class.queue)
+        .to eq BackgroundMigrationWorker.sidekiq_options['queue']
+    end
+  end
+
   describe '.steal' do
-    it 'steals jobs from a queue' do
-      queue = [double(:job, args: ['Foo', [10, 20]])]
+    context 'when there are enqueued jobs present' do
+      let(:queue) { [double(:job, args: ['Foo', [10, 20]])] }
 
+      before do
         allow(Sidekiq::Queue).to receive(:new)
-        .with(BackgroundMigrationWorker.sidekiq_options['queue'])
+          .with(described_class.queue)
           .and_return(queue)
+      end
 
+      it 'steals jobs from a queue' do
         expect(queue[0]).to receive(:delete)
 
         expect(described_class).to receive(:perform).with('Foo', [10, 20])
@@ -17,12 +27,6 @@ describe Gitlab::BackgroundMigration do
       end
 
       it 'does not steal jobs for a different migration' do
-      queue = [double(:job, args: ['Foo', [10, 20]])]
-
-      allow(Sidekiq::Queue).to receive(:new)
-        .with(BackgroundMigrationWorker.sidekiq_options['queue'])
-        .and_return(queue)
-
         expect(described_class).not_to receive(:perform)
 
         expect(queue[0]).not_to receive(:delete)
@@ -30,6 +34,7 @@ describe Gitlab::BackgroundMigration do
         described_class.steal('Bar')
       end
     end
+  end
 
   describe '.perform' do
     it 'performs a background migration' do