add testing method shift_and_perform

lib/sidekiq/testing.rb

@@ -87,6 +87,12 @@ module Sidekiq
           new.perform(*job['args'])
         end
       end
+
+      # Pop out a single job and perform it
+      def shift_and_perform
+        job = jobs.shift
+        new.perform(*job['args'])
+      end
     end
 
     class << self

test/test_testing.rb

@@ -119,6 +119,17 @@ class TestTesting < Minitest::Test
       StoredWorker.clear
     end
 
+    it 'shift_and_perform runs only one job' do
+      assert StoredWorker.perform_async(:first)
+      assert StoredWorker.perform_async(:second)
+      assert_equal 2, StoredWorker.jobs.size
+
+      StoredWorker.shift_and_perform
+      assert_equal 1, StoredWorker.jobs.size
+
+      StoredWorker.clear
+    end
+
     class FirstWorker
       include Sidekiq::Worker
       class_attribute :count