Add specs for stage update worker

spec/workers/stage_update_worker_spec.rb

+require 'spec_helper'
+
+describe StageUpdateWorker do
+  describe '#perform' do
+    context 'when stage exists' do
+      let(:stage) { create(:ci_stage_entity) }
+
+      it 'updates stage status' do
+        expect_any_instance_of(Ci::Stage).to receive(:update_status)
+
+        described_class.new.perform(stage.id)
+      end
+    end
+
+    context 'when stage does not exist' do
+      it 'does not raise exception' do
+        expect { described_class.new.perform(123) }
+          .not_to raise_error
+      end
+    end
+  end
+end