Add acceptance test

spec/acceptance/postfix_spec.rb

+require 'spec_helper_acceptance'
+
+describe 'postfix class' do
+
+  context 'default parameters' do
+    it 'should work idempotently with no errors' do
+      pp = <<-EOS
+        class { 'postfix': }
+      EOS
+
+      # Run it twice and test for idempotency
+      apply_manifest(pp, :catch_failures => true)
+      apply_manifest(pp, :catch_changes  => true)
+    end
+
+    describe package('postfix') do
+      it { is_expected.to be_installed }
+    end
+
+    describe service('postfix') do
+      it { is_expected.to be_enabled }
+      it { is_expected.to be_running }
+    end
+  end
+end

spec/spec_helper_acceptance.rb

+require 'beaker-rspec'
+require 'beaker_spec_helper'
+
+include BeakerSpecHelper
+
+hosts.each do |host|
+  install_puppet_on host
+  install_package host, 'git'
+  install_package host, 'tar'
+  install_package host, 'sudo'
+  on host, 'rm /usr/sbin/policy-rc.d || true'
+end
+
+RSpec.configure do |c|
+  module_root = File.expand_path(File.join(File.dirname(__FILE__), '..'))
+  module_name = module_root.split('-').last
+
+  # Readable test descriptions
+  c.formatter = :documentation
+
+  # Configure all nodes in nodeset
+  c.before :suite do
+    # Install module and dependencies
+    puppet_module_install(:source => module_root, :module_name => module_name)
+    hosts.each do |host|
+      BeakerSpecHelper::spec_prep(host)
+    end
+  end
+end