Add test example for after branch delete service

spec/services/after_branch_delete_service_spec.rb

+require 'spec_helper'
+
+describe AfterBranchDeleteService, services: true do
+  let(:project) { create(:project) }
+  let(:user) { create(:user) }
+  let(:service) { described_class.new(project, user) }
+
+  describe '#execute' do
+    it 'stops environments attached to branch' do
+      expect(service).to receive(:stop_environments)
+
+      service.execute('feature')
+    end
+  end
+end